Using retrospective data from the Urban Integration Survey conducted in 2001 in Nairobi, Kenya, on a sample of 955 women and men aged 25–54, this paper compares factors influencing entry into union formation for men and women. The analysis uses event history methods, specifically Cox Proportional Hazards regression, stratified by age cohort and run separately by sex. The results indicate that delay in union formation is more pronounced for women than for men. Cohabitation without formal marriage is the prominent form of union, especially among the younger generation, and appears to have increased. For men, the timing of union is more dependent upon human capital acquisition than on cultural factors. These findings show that the marriage search model, which was first applied in Western countries, can also hold in cities of developing countries. Nonetheless, neither the search model nor the integration or the independence models apply to women's union formation, which very few exogenous factors can explain.