Previous research on the difference in terms of lexical transfer between CLIL (Content and Language Integrated Learning) and non-CLIL students has revealed that CLIL students produce fewer lexical transfer errors than non-CLIL students. This study aimed at comparing the lexical transfer production of two groups of students (CLIL and non-CLIL) and determining whether language proficiency was a key factor when predicting differences between both groups (Bruton, 2011a). The sample for the research consisted of 36 students in grade 7. Subjects' language proficiency was assessed through the English Unlimited Placement Test, whereas their lexical transfer production was tested through a written composition in English. The results indicated that there was no significant difference in language proficiency between both groups and that the difference in lexical transfer lied in the influence exerted by a few participants on the results.