This project investigated vocabulary learning from a sociocultural perspective--in particular, the way in which lexical knowledge was mediated in Spanish second language (L2) learners' and native speakers' (NSs') interactions. Nine students who were enrolled in an advanced conversation course completed an oral portfolio assignment consisting of five conversations with an NS. In addition to recording their conversations, learners transcribed and analyzed their interactions. At the end of the semester, each learner was assessed using a tailor-made posttest targeting the lexical language-related episodes from his or her individual portfolio. Findings indicated that NSs provided lexical mediation and that mediated terms were later recalled by learners, thus demonstrating that vocabulary learning had taken place. However, learners' and NSs' behavior varied from dyad to dyad regarding their level of engagement and the quantity and quality of mediation they provided and received, which had an impact on final learning outcomes. Pedagogical implications include the application of the oral portfolio activity for vocabulary learning in L2 classrooms.