Since the electoral victory of Evo Morales and his Movimiento Al Socialismo (MAS) in December 2005, a lot has been written about the first indigenous president in Bolivia. However, less attention has been paid to study his “political instrument” (i.e. MAS). The present paper intends, therefore, to analyse qualitatively the internal organization of MAS with an emphasis on a comparative perspective between the party before and after 2004. Using participant observation and in-depth interviews, the aim is examine the internal change of the organization taking into account dimensions such as candidate selection, policy formulation or internal control. This will illuminate an increasing tendency of institutionalizing centralized party structures and a simultaneous reduction of the impact of informal politics within the party. In this framework, we can point to some challenges of a process of institutionalization, which is understood here in the terms of Randall and Svåsand (2002:12) as a “process in which a party becomes established both in terms of integrated patterns of behaviour and of attitudes, or culture”. The case of MAS shows us that there are other paths towards the establishment of party structures than described in the traditional model with its emphasis on a formalised structure. Notwithstanding these processes, the principal argument of this paper is that the internal structure of MAS is quite different from a traditional Bolivian party owing to its decentralization and enormous influence of grass-root and informal organizations. For this reason, a certain degree of institutionalization could contribute to that MAS will have a positive impact on the quality of democracy without reproducing the deficiencies of other Bolivian parties. A socio-organizational diagnostic of MAS, including the identification and composition of its actors, its geographical coverage and influence, identification of its socio-cultural values in its discourse, will contribute to draw a picture of the evolution of new forms of participation and societal control within a process of strengthening democracy.