Veto Players and the Enforceability of Side-Payments.The Swiss Decision Making Process on the Flanking Measures to the Agreement on Free Movement of Persons with the European Union Once an agreement is signed at the international level, it can no longer be modified during the domestic decision-making process. However, it is possible to compensate potential veto players by a purely domestic legislative act. Under which circumstances do such side-payments make sense, and what is the role played by political strategies? In order to be compensated, an actor needs to be considered a veto player. Furthermore, the defenders of the international agreement must have a sufficiently large interest in its ratification to be willing to offer compensations. These basic conditions fulfilled, the existence or not of side-payments depends on the strategic interactions between the political actors. In the last ten years, Swiss citizens had to vote twice on an international agreement establishing free movement of persons. In both cases, the unions asked for flanking measures; while they failed in their attempt in the context of the European Economic Area, they succeeded in the case of the bilateral agreements. This difference in outcome is all the more surprising since in both cases the unions were in a veto player position and the export oriented economy had a strong interest in the ratification of the agreements. This article explains the different fate of the unions’ claims by stressing the role of changes in actor strategies. The credibility of the threat to block the decision-making process and the succession of the game sequences are in this perspective of paramount importance. By using strategically the veto threat, domestic groups such as unions are able to benefit from internationalisation.