Eastern enlargement is a process in which the very identity of the European Union is under construction, with its constitutive principles and values being challenged. Policy choices of the member governments participating in this process can not exclusively being explained by rational choice assumptions, but a sociological approach to the issue also becomes necessary. Through the analysis of the Spanish government's policy on enlargement, this paper aims to show how policy preferences might not only respond to a cost-benefit calculus and how policy contents might develop endogenously during the process of policy formation in the EU. In this process there is room for member states to pursue their goals and search a collective decision that better accommodates their self-interest. However, deliberation also matters, and governments have to justify their actions in terms considered legitimate by all according to the constitutive principles and values of the EU polity.