Abstract A general advantage in proficiency has been repeatedly reported for learners receiving Content and Language Integrated Learning (CLIL) when compared to learners who only receive English lessons. However, fine-grained studies addressing the aspects which make up this general advantage are still scarce. Within this context, this paper concentrates on the morphosyntactic development of two groups of (Basque-Spanish) adolescents learning English in high-school over a two-year period. One group (n= 15) received CLIL instruction and English classes while the other group (n=11) only received English classes. The results indicate a clear advantage for the CLIL group, which seems to be at a further developmental stage. Nonetheless, both groups improve over the two years and, unlike previous claims in schools, no signs of fossilization are found although inaccuracies in inflection still exist. In light of these results, the value of increasing exposure in the form of CLIL lessons in high-school is discussed.