In most European metropolitan areas of Europe – like Brussels or Luxembourg – social and spatial inequalities have been rising since the last decades. Mechanisms of selective inward and outward migration have been playing a major role of both reinforcing existing segregation structures and initiating gentrification and spatial dispersion. We analyse the dynamics caused by both residential mobility and international immigration and look at their effects on the rise of disparities, by studying the diffusion of different social groups in the metropolitan areas. Against the background of post-fordist competitive mechanisms we will thus question the role of residential movements as a motor of social composition and spatial structure in Brussels and Luxembourg.