This paper reports on a follow-up study of a Content and Language Integrated Learning (CLIL) pilot experience with bilingual postgraduate engineering students at a Spanish university. It aimed at examining learners' gains in listening and grammar skills after a CLIL course in English for a semester, in particular whether students' listening and grammar skills were affected similarly and whether participants' proficiency level played a role. Paired-sampled t-tests showed the difference between the mean scores in the pre-and post-listening test was significant but it was not for the pre-and post grammar tests. When students were distributed into three groups on the basis of their pre-test scores, a repeated measures ANOVA showed that less proficient students obtained higher gains in listening and grammar skills than more proficient students.