This article explores the effect of CLIL (Content and Language Integrated Learning) and non-CLIL approaches (English as a Foreign Language) on written production in L2 of two groups of upper secondary students (grade four, age sixteen). To address this concern, we focus on a comparative study based on a content- and language-integrated learning (CLIL) group and an English as a foreign language (EFL) or non-CLIL group and their competence in written English. This study aims toward an error analysis based on Canale and Swain’s theoretical approach, defined as an underlying system of knowledge in which three fields can be differentiated: discourse competence, linguistic competence, and strategic competence. The written productions of both groups were measured along these three dimensions. The findings show that the CLIL group outperforms the non-CLIL group and that the latter tend to resort to the L1 to solve its lack of competence with the L2.