The subject of this study are the Bolivian immigrants in São Paulo; at the time of the study (2008-2010) the biggest immigrant group in the city with more than 200.000 people. Based on linguistic data I collected empirically, a holistic approach towards the linguistic processes involved in migration is developed and tested to answer the central questions for the Bolivian immigrants in São Paulo: Who speaks Portuguese? In which way? Why? Based on a sociological excurse that claims that assimilation processes towards social organization inside a differentiated society are valid to explain the behavior of migrants, my study identifies accommodation as the decisive process with regard to language contact due to migration: It explains not only the choice of language maintenance resp. language learning as consequence of individual attitude and the social environment, but also the use of certain socio- indexical features. They are employed in order to accommodate to the variety that is perceived as socially successful in the social network the migrants live in. With regard to social factors influencing these networks, the speakers’ attitude and the quantity and quality of Portuguese-speaking contacts have to be considered. Further linguistic processes that have to be taken into account in this context are L1-transfer and simplification. I show that these processes are conditioned and influenced by accommodation as well, i.e. they underlie social constraints. The social constraints of Bolivian immigrants in São Paulo are defined by factors like the size and heterogeneity as well as the strong social stratification of the population of the city and of the Bolivian community. Methodologically, the speech of the Bolivians themselves as well as data about their attitudes and their social networks, have required qualitative ethnographic research: The first part consists of the composition and analysis of a corpus of 28 sociolinguistic interviews with Bolivian immigrants living in São Paulo that were conducted in 2008, giving a detailed presentation of their social characteristics such as age, duration of stay and profession. Since social indexicality is based on social convention, a further quantitative study that included the social stratification of São Paulo’s population could confirm whether the linguistic strategies employed by the Bolivians are actually successful. Therefore, an online questionnaire based on the analysis of the qualitative data was developed and distributed between Paulistanos in 2010, resulting in the statistical evaluation of 100 responses.