This study investigates the differential effects of two learning contexts, formal instruction (FI) and content and language integrated learning (CLIL), on the written production skills of intermediate-level Catalan Spanish adolescent learners of English as a foreign language. Written samples elicited through a composition at two data collection times over one academic year were quantitatively and qualitatively assessed for complexity, accuracy, and fluency and for task fulfilment, organization, grammar, and vocabulary, respectively. Based on the findings, the superiority of CLIL cannot be confirmed: although improvement in the case of the FI + CLIL group is shown, results were only significant in the domain of accuracy.