The aim of this article is twofold: to assess the effectiveness of a CLIL (content and language integrated learning) module on affective factors (motivation and self-esteem), and to test the purported blurring effect of CLIL on gender differences in foreign language learning. Forty-six students in their fourth year of compulsory secondary education were administered a background questionnaire, a questionnaire on motivation and self-esteem, and a vocabulary test. Participants' learning of vocabulary was tested on a pre-test, immediate post-test and delayed post-test design. Results revealed that CLIL can help reduce gender differences in motivation. Similarly, vocabulary tests indicated that the CLIL module had a similar positive effect on both male and female students' learning of the technical content-related vocabulary.