Previous studies on social integration of immigrants were predominantly of static nature. This article provides a dynamic account by distinguishing differences in social integration that develop shortly after immigrants’ arrival in the host country from changes with length of stay. We examine contacts in leisure time between natives and non-western immigrants in the Netherlands. Theories from research on ethnic intermarriage are applied to these more common forms of contact. We use pooled cross-sectional survey data from 1988–2002. The results show that on average social integration increases with length of stay. Ethnicity, migration motive and home country education account for differences in integration that develop shortly after arrival and are maintained or even increased with length of stay, while the size of the immigrant group matters mainly at entry and then loses its influence over time. Age at migration exclusively explains differences in social integration that develop with length of stay.