Gap-Filling in Investment Protection Treaties: Dual Nationality and Treaty Silence in UNCITRAL Arbitration – A comment on Romero v. Ecuador

Investment treaties are necessarily incomplete. Not every contingency can be anticipated, and silence is an inherent feature rather than a defect. Some omissions are more relevant than others. There are cases in which gap-filling can determine the outcome of a dispute, such as when analyzing if dual nationals are entitled to bring arbitration against their host State.

Romero v. Ecuador, a recent arbitration conducted under the 1976 UNCITRAL Rules (“UNCITRAL Rules”), illustrates how treaty silence on the treatment of dual nationals may give rise to different interpretative approaches, as reflected in the award and the dissenting opinion. In their award rendered in December 2025, the majority of the tribunal declined jurisdiction after concluding that the parties had not agreed to protect claims brought by dual nationals, even if the treaty has no express provision to that effect. The dissent reached the opposite conclusion: absent a prohibition, dual nationals are not barred from initiating arbitration proceedings against their host State.

The dispute

Santiago Romero Barst and María Auxiliadora Rodríguez (“Claimants”) brought an arbitration against the Republic of Ecuador under the international investment treaty concluded in 2001 between Italy and Ecuador (“BIT”). The Claimants alleged breaches of the fair and equitable treatment standard, denial of justice, and unlawful expropriation following Ecuador’s nationwide ban on casinos and gambling in 2011. The proceedings were administered by the Permanent Court of Arbitration (“PCA”), with Paris as the seat of the arbitration.

The Claimants are Ecuadorians who later acquired the Italian nationality. Both were born and have resided in Ecuador throughout their lives. Ms. Rodríguez obtained Italian nationality in 2002 through jure sanguinis, with retroactive effect to birth, while Mr. Romero acquired Italian nationality in 2007 through jure matrimonii.

The dispute arose from a long-standing involvement by the Claimants in the local gaming industry. Gaming activities had been lawful for several decades and were expressly recognized as legitimate tourism activities under Ecuadorian legislation. The Claimants operated multiple casinos through locally incorporated companies, which were registered as national investments. Nonetheless, after the referendum in 2011, all gambling activities in Ecuador came to a halt.

The arbitration commenced in 2022, more than a decade after the ban took effect. The BIT provides investors with several dispute settlement options, including arbitration under the ICSID Convention and ad hoc arbitration under the UNCITRAL Rules. The Claimants chose an ad hoc arbitration under the UNCITRAL Rules, which, unlike ICSID Rules, contain no express limitation on claims by home-host State dual nationals.

The award and the dissenting opinion

Ecuador challenged the jurisdiction of the tribunal claiming that the BIT does not afford protection to home-host State dual nationals. The proceedings were bifurcated, and Ecuador’s challenge was successful. The majority concluded that Ecuador had not consented to UNCITRAL arbitration in disputes brought by Ecuadorian nationals. The tribunal based its conclusion on the BIT’s Protocol, which directs to the application of the “principles of international law recognized by the two Contracting Parties”. The majority concluded that one of those principles is the doctrine of dominant and effective nationality. Since the Claimants have a substantial relationship with Ecuador, they are not afforded the protections provided in the BIT.

The dissenting arbitrator disagreed with the tribunal’s reliance on the doctrine of dominant and effective nationality. That doctrine, in his view, does not constitute either a rule of customary international law or a principle of international law applicable to the case. He further emphasized that neither the BIT nor Ecuadorian domestic law treats dual nationality as a bar to access to investment arbitration under the UNCITRAL Rules. Finally, the dissenting arbitrator stressed that any limitation on claims brought by dual nationals must be explicit and cannot be presumed, applied by analogy, or derived from extensive interpretation.

Opt-in or opt-out?

The majority opinion and the dissent reflect two distinct approaches to treaty silence in investment arbitration about the rights of dual nationals to bring claims against their host State. Whereas the majority construed silence in the BIT as permission to rely on principles of international law to delineate the scope of the State’s consent to arbitration, the dissent adopts a different starting point. The dissent treats silence as requiring restraint, on the premise that limitations on access to international investment arbitration must be explicit. In that view, the absence of a prohibition on claims by home-host State dual nationals cannot be filled through analogy or the importation of external doctrines. This approach aligns with the structure of investment arbitration under the ICSID Convention, where the exclusion of home-host State dual nationals follows from an express provision rather than from interpretative supplementation.

Read together, the award and the dissent illustrate a broader question in UNCITRAL-based investment arbitration: whether treaty silence operates as an opt-in mechanism, allowing tribunals to supplement the text with external sources, or as an opt-out mechanism, requiring express language to restrict jurisdiction.

Recent arbitral practice confirms that no uniform approach has emerged regarding dual nationals in non-ICSID arbitration. The jurisprudence under the Spain–Venezuela BIT is particularly illustrative of this divergence. In Diamante Trading Ltd. et al. v. Venezuela (discussed here), the tribunal concluded that dual nationals had standing to initiate arbitration against Venezuela and declined to apply the doctrine of dominant and effective nationality as a limiting principle. This outcome aligns with the earlier decision in Serafín García Armas v. Venezuela (discussed here), where a tribunal under the same BIT also affirmed jurisdiction over claims brought by dual nationals by emphasizing that, in the absence of clear textual exclusion, the treaty’s provisions govern and should not be supplemented with principles of international law.

By contrast, other UNCITRAL tribunals under the same BIT have adopted a different interpretative route. In Santamarta v. Venezuela (discussed here), the tribunal reasoned that, given the BIT’s silence on dual nationality, it was necessary to resort to other rules of international law and, applying the doctrine of dominant and effective nationality, concluded that the claimant’s Venezuelan nationality was dominant and declined jurisdiction. Santamarta thus joined a group of decisions in which the dominant and effective nationality test has been applied to reject jurisdiction over dual nationals under the Spain–Venezuela BIT, including Fernando Fraiz v. Venezuela, Manuel García Armas et al. v. Venezuela and Enrique Heemsen v. Venezuela (discussed here).

Another recent example arose under the North American Free Trade Agreement (“NAFTA”) in Alicia Grace et al. v. United Mexican States (discussed here). Confronted with treaty silence on dual nationality, the tribunal applied the dominant and effective nationality test and declined jurisdiction over certain dual nationals whose effective nationality was Mexican. A similar approach was adopted in Alberto Carrizosa Gelzis et al v. Colombia (discussed here), where a UNCITRAL tribunal likewise rejected jurisdiction after finding that the claimants’ dominant and effective nationality corresponded to the respondent State.

The view of the courts of the seat

The treatment of treaty silence in Romero v. Ecuador acquires additional significance in light of the seat of the arbitration.  In several decisions, French courts have declined to endorse the insertion of implied limitations into investment treaties that are silent on dual nationality. In cases such as Aboukhalil v. Senegal, Serafín García Armas v. Venezuela, and Dangelas v. Vietnam, the Paris Court of Appeal considered that, absent explicit treaty provisions, arbitral tribunals could not exclude home-host State dual nationals by applying an effective nationality test or by analogy to rules of diplomatic protection. In those decisions, the courts reasoned that where a treaty draws no differentiation, none should be read into it through interpretation.

The tribunal acknowledged those precedents and the authority of the French courts to review the award but considered the circumstances to be materially different. The tribunal emphasized that the Italy-Ecuador BIT expressly directs tribunals constituted under the UNCITRAL Rules to apply principles of international law. On that basis, the tribunal viewed reliance on the doctrine of dominant and effective nationality as consistent with the interpretative framework agreed by the States. It further relied on the absence of any substantive connection between the Claimants and Italy.

The requirement of a substantial connection

The jurisdictional issues raised in Romero v. Ecuador also resonate with another recent UNCITRAL arbitration arising from Ecuador’s 2011 measures affecting the gaming sector. In Lynton Trading v. Ecuador, brought under the Ecuador-United States BIT, the tribunal did not address questions of dual nationality, but declined jurisdiction on the grounds of a denial of benefits clause, since the claimant failed to demonstrate a substantial business activity in the United States. Although the jurisdictional filters applied in both cases differ, they reflect a shared concern with identifying a genuine and substantive connection between the claimant and the State through which treaty protection is sought.

Conclusion

Romero v. Ecuador gives rise to several comments regarding the issue of treaty silence on dual nationality in UNCITRAL investment arbitration. First, the case raises the question of whether silence on dual nationality should be understood as operating on an opt-in or opt-out basis. In the context of UNCITRAL arbitration, this question may ultimately be clarified, at least in this case, through judicial review by the courts of the seat. Second, the case reflects a broader tendency in investment arbitration to examine whether a claimant maintains a substantial connection with the State through which treaty protection is sought. Finally, as illustrated by both arbitral practice and judicial review, jurisdictional outcomes in cases involving treaty silence turn on the interpretation of the specific treaty at issue, which explains why no uniform criterion has emerged, and why none should necessarily be expected.