Condom use within steady and casual sexual relationships was examined among 14-16 year old Dutch adolescents from secondary school (N = 140). It was hypothesised that among adolescents sex and subsequently condom use with casual sex partners is less likely to be considered in advance, more context-dependent and less habitual; whereas the opposite is true for steady relationships. Therefore, preparatory behaviours (buying and carrying condoms and communicating about condom use) were expected to mediate the intention-behaviour relation in the context of steady relationships, but not in the context of casual sex. Results confirmed that condom use with steady sex partners was explained by preparatory behaviours, habits, and to some extent, behavioural willingness, and that preparatory behaviours mediated the intention-behaviour relationship. Condom use with casual sex partners was predicted by risk willingness and intentions, without any mediation by preparatory behaviours. The results indicate that it is essential to increase awareness among adolescents that unexpected sexual situations may occur and to train them to take preparatory actions.