This paper examined disagreement in two sets of data in the context of service encounters: problem-solving interactions (doctor-patient communication) and purchase-oriented encounters (pharmacies) from a cross-cultural perspective (Spanish-British English). We proposed assertiveness, a term that refers to both socio-psychological and linguistic features of communication, as a concept that may help understand disagreement. To this end, this study explored, on the one hand, frequency and types of disagreement in 160 British and Spanish service encounter interactions (SEIs, henceforth), in order to understand degrees of assertiveness, as well as the difficulty to grasp motivations for disagreement. On the other hand, five case studies were examined to unravel the social meanings attached to disagreement. The results showed that not in all cases Spanish interlocutors are more assertive than British interlocutors, that social meanings are not stable within the same genre and that linguistic choices may be linked to psychosocial motivations, such as assertiveness.