This study examines the causal effects of the Dutch language proficiency of immigrants on their labour market and social integration outcomes. Firstly, we estimate determinants of the proficiency of Dutch language using rich surveys. Subsequently, we examine the role of language proficiency in shaping the self-assessed integration outcome and feeling-Dutch in addition to two major structural determinants of integration, employment and income. The analysis shows that endogeneity of language skills masks a substantial part of language effects. Once accounted for endogeneity, effects of the Dutch language proficiency on social and economic integration of immigrants are more than double of the estimates ignoring endogeneity.