To what extent do voluntary organizations like sports, leisure, and neighborhood associations provide a platform where ethnic groups mingle and ethnic boundaries are overcome? This study uses unique panel data from the Netherlands Longitudinal Life Course Study (NELLS) to shed light on the integrative power of voluntary associations. I investigate decisions to join and leave associations of different ethnic composition, as a member or a volunteer, among individuals of Turkish, Moroccan, and native Dutch origin. In general, all ethnic groups are equally likely to join voluntary organizations, but ethnic minorities are more likely to leave than are Dutch natives, even after accounting for relevant sociodemographic characteristics. This alone explains ethnic minorities’ lower involvement rates. Moreover, joining decisions are characterized by strong ethnic sorting across organizations of different ethnic composition: people are much more likely to join associations containing fewer ethnic out-group members. This limits the potential of voluntary associations as pathways to social integration. In contrast, once the initial hurdle of getting involved has been taken, people are no more likely to disengage from organizations with more ethnic out-group members. Inter-ethnic neighborhood contact and the local supply of involvement opportunities are most influential in explaining the strong sorting tendencies in people’s joining decisions.