In line with van Kampen et al.'s (2018) research about specialist and practitioner perceptions on the goals for CLIL in the Netherlands, the present study addresses Hüttner, Dalton-Puffer and Smit's (2013) call for investigating teachers' beliefs about CLIL in European countries like Spain where this teaching approach is highly institutionalized. Unlike the aforementioned studies, though, ours focuses on novice subject teachers in Primary education. More specifically, it links teacher cognition and pre-service or initial teacher training with the aim of exploring the extent to which student teachers' beliefs mediate their education. The analysis of how these student teachers understand the aims and language principles of CLIL unveils the role that a set of monolingual ideologies (e.g. "the English only policy") play in the conceptualization and consequent acceptance or disapproval of this teaching approach. As we also pay heed to the impact of tertiary instruction on participants' beliefs, implications for CLIL teacher preparation programmes are discussed.