Social scientists generally agree that better individual economic prospects enhance the probability of marriage for men, whereas there are conflicting views with regard to women. Moreover, it is argued that cohabitation does not require as strong an economic foundation as marriage. The aim of this study, which was based on Finnish register data, was to find out how the socio-economic resources of young adults affect first-union formation, and whether the effects vary by sex or union type. The results show that high education, labour-force participation, and high income seem to promote union formation. The findings are similar for women and men, which is plausible given the comparatively gender-egalitarian societal context. Similar factors encourage entry into both union types, although the union-promoting effects of university-level education and stable employment are stronger in the marriage models, suggesting that long-term prospects are more important when marriage is contemplated.