This study examines the impact of the decision-making rules, procedures and practices of the European Union on the ability of small Member States to influence Treaty negotiation  outcomes, and assesses the causality of this influence in Treaty-revision. Within Treatymaking  processes, actor influence is here expected to vary according to the institutional preconditions, and small states are presumed to benefit from particular type of decisionmaking  rules to the disadvantage of others. The fundamental aim of the study is therefore to  investigate the conditions under which small state influence increases in European Union  Treaty-negotiations. To explain this puzzle, a distinction is made between two types of  Treaty-making processes, those of the Intergovernmental Conferences and the Convention,  which allows for subsequent comparisons between the decision-making rules of unanimity and restricted consensus. In order to empirically test the underlying hypotheses, explicit units of observation are chosen from the IGCs of Amsterdam, Nice and 2003-04, and the Convention on the Future of the EU. In-depth comparisons are made between four small Member States – Belgium, Denmark, Finland and Ireland – and their de facto influence is process-traced through three substantial issues of the institutional reform: the composition of the Commission, the extension of qualified majority voting and the reform of Council Presidency. The empirical analysis focuses on both informal and formal levels of decisionmaking dynamics, and a further analytical distinction is made between bargaining and deliberation modes of conflict-resolution. Drawing initially on theories of rational choice institutionalism (RCI) and liberal intergovernmentalism (LI), the unanimity rule as applied in the IGCs is expected to strengthen the formal position of an individual small state by providing, respectively, a veto-right for each negotiator and promoting asymmetric interdependency. The empirical findings confirm the major underlying hypothesis concerning the correlation between the adopted decision-making rules and the small states’ impact on distributional outcomes in the EU Treaty-amending negotiations on the one hand, and the superiority of the unanimity rule for small states on the other. Yet, a couple of additional key success factors – other than can be explained exclusively along the conjectures of RCI or LI schools – are also identified in the study.