Heritage Language Instruction (HLI) is a resource used in many immigration-receiving countries that allows students with an immigrant background to continue to be in contact with their Heritage Language (HL). However, many of the psycholinguistic effects of this instruction are still unknown. This study aims to provide an in-depth view of the effects of HLI on the development of the HL and the languages of the host country. For this purpose, the sample consisted of 108 students (20 HL learners, 88 controls) of Romanian origin who had emigrated to Catalonia. Their language skills in the phonetic, orthographic, morphosyntactic, lexical, and semantic domains were analysed using linear regression statistics and covariance analysis. The results showed that HLI exerted a mitigating effect on HL attrition, as well as a significant improvement of the linguistic competencies in the autochthonous languages. These results indicate that HLI is an effective tool in the psycholinguistic development of immigrant students, capable of facilitating their academic and social performance.