Utilizing longitudinal population register data from Finland, this study examines the influence of exogamy on transitions within and from first unions. The aim is to assess how ethno-linguistically mixed unions, consisting of Swedish speakers and Finnish speakers, differ from endogamous unions with respect to various transitions in the family formation process subsequent to entry into childless cohabitation. We find evidence of notable selection. The proportion of endogamous relationships increases during the course of the courtship process, and this selection is primarily driven by a higher separation risk of ethno-linguistically mixed unions. The stages in family formation consequently seem to work as a social filter, where the exogamy effect on the dissolution risk is particularly strong for couples who have come a long way in the process.