While Southern European countries have pursued a series of pension reforms since the early 1990s, significant variation arises across them. Focusing on the concept of political replacement risk (the probability of a government being electorally punished for pursuing a given policy) and the changes in the labor movement's organizational structure, this article seeks to elucidate the differences in reform outcomes, in Italy, Greece, Portugal and Spain. Our analysis shows that significant reforms are implemented when governments face a high political replacement risk and the labor movement has undergone changes in its structure. By contrast, governments facing a stronger labor movement will generally be less effective at passing significant reforms, unless they can secure a strong support over the necessity to implement reforms