This article addresses the issue of underrepresentation or avoidance of colloquial words in a cross-sectional corpus of advanced French interlanguage (IL) of 29 Dutch L1 speakers and in a longitudinal corpus of 6 Hiberno-Irish English L1 speakers compared with a control group of 6 native speakers of French. The main independent variable analysed in the longitudinal corpus is the effect of spending a year in a francophone environment. This analysis is supplemented by a separate study of sociobiographical and psychological factors that affect the use of colloquial vocabulary in the cross-sectional corpus. Colloquial words are not exceptionally complex morphologically and present no specific grammatical difficulties, yet they are very rare in our data. Multivariate regression analyses suggest that only active authentic communication in the target language (TL) predicts the use of colloquial lexemes in the cross-sectional corpus. This result was confirmed in the longitudinal corpus where a t-test showed that the proportion of colloquial lexemes increased significantly after a year abroad.