Abstract CLIL studies on language specific areas such as morphosyntax are still quite limited, particularly those with young learners. Likewise, the pseudo-evolution of morphosyntactic aspects across age and proficiency deserves particular attention. This paper will fill these gaps by investigating production accuracy and syntactic complexity in two different age/proficiency groups of CLIL primary school learners (grade 4 and 6). In terms of accuracy, L1 Spanish transfer effects associated with the pro-drop parameter (i.e. subject omission and subject-verb inversion), and the third person singular -s morpheme omission will be explored. Syntactic complexity will be operationalised through the production of simple and complex clauses. The findings obtained align with previous research in that the accumulated hours of CLIL + EFL exposure by grade 6 seem to positively affect the development of complexity measures. However, the lack of progress in the case of the rest of the features examined (i.e. subject omission, inversion of the subject and the third person singular -s omission) calls for the incorporation of focus-on-form components in CLIL programmes.