Recent second language (L2) acquisition research has proposed that purely syntactic features are easier to acquire and less vulnerable than ones involving the interfaces (Sorace, 2004; Serratrice et al. 2004). The present paper addresses this issue by investigating the acquisition of the Spanish personal preposition a in English L2 learners of Spanish. The distribution of a in direct object NPs relates to the specificity/definiteness of the NP, the animacy/agentivity of the subject, and verb semantics (Torrego 1998; Zagona 2002). 33 English L2 learners of Spanish of different proficiency levels, and 14 Spanish controls participated in an acceptability judgement task. The results showed significant differences between native speakers and L2 learners of all proficiency levels, who performed at chance, and support the claim that L2 learners have difficulties acquiring structures involving the syntax/semantics interface. However, the advanced learners showed sensitivity to the least complex condition providing evidence that interface phenomena may be acquirable.