The research investigates the effects of two learning contexts, content and language integrated learning (CLIL) and traditional learning (non-CLIL), upon content-specific outcomes, an aspect less explored than language-specific achievements in CLIL. Specifically, the study provides an interdisciplinary analysis of English CLIL applied to Physics in Italian high school. Two different levels of student competence are examined: selecting answers for content-specific issues, and content-related argumentative skills, in order to measure how learners comprehend and discuss content. The comparison between CLIL and non-CLIL classes in pretest, posttest and delayed posttest accounts for content assimilation and retention. Findings show that CLIL students significantly outperform non-CLIL students in both levels of competence in posttest and even more so in delayed posttest, a difference which emerges in terms of mean and coefficient of variation. Furthermore, feedback questionnaires display enhanced motivation, a well recognised influential benefit of CLIL in language-specific learning. Such results have pedagogical implications and may contribute to a better understanding of the correlation between language, content and motivation in CLIL.