The present study aims at determining whether instruction in the form of explicit phonetic training and of implicit exposure to native input impacted Content and Language Integrated Learning (CLIL) students’ phonological awareness of the occurrence of English schwa in unstressed syllables of content words (bacon). Four intact CLIL groups were administered a perception task immediately before and after an intervention period of one month in which two groups underwent explicit instruction on the incidence of reduced vowels versus full vowels in English disyllabic words while another group was exposed to native input in their CLIL sessions. A fourth CLIL group with neither explicit intervention nor native teacher input served as control group. All four groups tended to judge both schwas and full vowels as correct in the pre-test, indicating that they were not knowledgeable of the general pattern of vowel reduction occurrence in unstressed syllables in English prior to intervention. In the post-test, the three experimental groups significantly improved their ability to identify full vowels as incorrect, the groups receiving explicit instruction exhibiting higher gains than the group which was implicitly exposed to native input.