This study examines the role of trainable intercultural personality traits in the widely assumed link between immigrants’ second language (L2) learning and their cultural integration in the host country. The research was based on data of temporary immigrants (sojourners), being international students who reside in the Netherlands and participants of a Dutch language course (total N = 163). Questionnaires were used to collect data at two time points (time lag: 3 months). Findings from multiple regression analyses reveal that an increase in L2 proficiency is related to a positive change in two indicators of cultural integration: identification with the host society and attitudes towards the host culture. An increase in L2 proficiency appears to be related to a positive change in terms of identification, regardless of sojourners’ personality traits. With respect to attitudes towards the host culture, however, we find that increasing L2 proficiency only goes together with a positive change in attitudes towards the host culture for sojourners with a high degree of social initiative. Another notable finding is that, in the time period that this study was conducted, sojourners with a high degree of openness became more positive in their attitudes towards the host culture, regardless of the progress they made in terms of L2 proficiency. Results from this study cautiously suggest the importance of social perceptual personality traits in the cultural integration of sojourners. In line with this, institutional policies on the L2 learning of sojourners, including the training of social perceptual personality traits, could be recommendable.