This paper investigates the extent of second language (L2) use in four cognitive domains including mental calculation, planning (action plans), note-taking, and shopping lists. Participants include 149 highly educated L2-competent sequential Polish–English bilinguals who relocated to the UK1 in early adulthood, and underwent processes of acculturation. The independent variables in this study include acculturation level, social network profile, predicted future domicile, and length of residence. The study employed both quantitative and qualitative approaches. Participants completed an online questionnaire and 14 were interviewed by the researcher. The study included the Complementarity Principle (CP) into the operationalisation and measurement of language use in bilinguals (Grosjean, 2010). The results show that acculturation level, social network profile, and predicted future domicile are strong predictors of the extent of L2 use in cognitive domains. Effects of context-specificity and language-dependence were also found, the latter specifically in the domain of mental calculation.