Urban research often considers densification from the perspective of sustainable development and social mix. This essay focuses instead on the social and political stakes involved in densification through the example of a large French metropolitan area. It shows that the densification policies put in place in the Lyon agglomeration cannot be said to succeed in breaking down the historical segregation between its residential and affluent western suburbs (banlieues) and its industrial and working-class eastern ones. The political manoeuvres executed by the institutions implementing densification, and the search for consensus characterizing France’s intercommunalities, block any possibility of redistributing functions and social classes at the metropolitan scale, and hence of ending the social specialization of Lyon’s suburbs. Moreover, municipalities subjected to pressure from suburban areas carefully assess the profile of residents selected to occupy new housing units—i.e. individuals already residing in the commune in the case of western suburbs, and middle-class individuals hailing from the eastern part of the agglomeration in the case of eastern suburbs. Densification does not foster social mix at the metropolitan scale, neither does it improve the housing conditions of disadvantaged populations.