The present study seeks to contribute new evidence on the effects of the CLIL approach (Content and Language Integrated Learning) on young EFL learners’ productive and receptive skills in a school set in Barcelona (Catalonia). Catalonia is a bilingual community in which both Catalan and Spanish are official languages and English represents the first foreign language included in the curriculum. For that purpose, two groups (n= 50 each) involving 100 bilingual Catalan / Spanish students aged 12 to 15 were analysed longitudinally over two academic years in two different types of exposure contexts: FI (Formal Instruction of English as a foreign language school subject, control group) and CLIL (English as medium of instruction when learning Science, experimental group). Data were elicited both for productive and comprehension skills and were statistically analysed quantitatively and also qualitatively using a posttest design at the end of each academic year. Results obtained confirm the effectiveness of the CLIL programme, however not in all domains and to the same degree as significant benefits did not accrue in all skills and measurements.