Throughout Europe, the average household size has decreased during the past decades. Part of this decline is due to changes in household composition. These are characterized by a shift from the extended family household to the nuclear family as the traditional household type. Several studies show that the nuclearization of the family is less prominent among migrant populations. For them, family ties are often of greater importance and the incidence of co-residing with family members and living in multi-generational households is found to be higher. Given that union dissolution rates vary greatly between native and non-native groups as well as between couples with at least one migrant partner, our central research question is whether aspects of family composition raise or lower union dissolution risks differently by ethnic composition of the couple. We expect certain aspects of family composition, such as having children and co-residing with relatives and non-relatives, to work as a promoting factor of union dissolution for Belgian native couples while they works as a protective measure for the (often more vulnerable) migrant couples. We investigate the association between family composition and union dissolution using Belgian data from the Crossroads Bank for Social Security for a sample of marriages and cohabitations formed between 1999 and 2003. The sample is disproportionally stratified by migrant status to include a sufficient number of relationships consisting of at least one partner of migrant background. Using discrete-time event history analysis we look whether different aspects of family composition raise or lower union dissolution risks. Results show that the impact of household composition differs by the ethnic composition of the couples. The negative impact of having (young) children in the household is strongest for non-European migrant couples and living with parents significantly raises union dissolution risks for mixed Turkish and Moroccan couples.