There is strong controversy over the application and effects of the Linguistic Immersion Policy in Catalonia, a policy that established that the Catalan language should constitute the only vehicular language during the different levels of compulsory schooling. While some of the effects of this policy have been determined, the evidence on other effects is scarce or non-existent. Within this group is the determination of the effect on academic performance of receiving schooling in a language other than the mother tongue. This paper analyses the performance of 15-year-old students as a function of their mother tongue. To do this, multivariant techniques are applied to the data provided by the assessments of PISA-2015. The results indicate that Spanish speakers obtain worse results than Catalan speakers all other variables been equal. The reduction in the level of competencies varies, in turn, among different subgroups of this collective.