Abstract Research on L3 acquisition has highlighted the importance of cross-linguistic influence in adult language acquisition. Specifically, the Typological Primacy Model (TPM) for L3 acquisition predicts full transfer from the most structurally similar background language at the initial stages of acquisition. However, it is unclear how language background might affect transfer. Furthermore, it is not known how applicable the TPM is to instructed L3 language acquisition. Consequently, this study examines Spanish/English bilinguals, their knowledge of mood distinctions in subjunctive-dominant contexts in Spanish and their transfer of that knowledge at the beginning stages of instructed L3 Portuguese acquisition. Data from eight L1 Spanish, 22 L2 Spanish, and 38 Spanish heritage bilinguals were analyzed to determine if the groups differed in their knowledge of mood distinctions, first in Spanish and subsequently in Portuguese. Results indicate a significant role of language background on all tasks.