This article uses micro-data from the Spanish National Immigrant Survey to investigate the impact of Spanish language proficiency on immigrants' earnings. The results, based on Instrumental Variables (IV), point to a substantial earnings return to Spanish proficiency, of approximately 27%. This figure varies largely across educational groups, with high-qualified workers earning a premium of almost 50%. This conspicuous complementarity between formal qualifications and language skills poses a challenge for traditional language training policies, for these typically neglect the immigrants' heterogeneous educational background.