This study examined incidental receptive and productive vocabulary gains within conversation-class interactions. Eleven Mexican learners of English attended four videotaped conversation lessons where 40 target words were incorporated in different types of exposure. Stimulated recall interviews with students highlighted the effect of cognates, learners' access to passive vocabulary, and use of their vocabulary knowledge in learning related words. Posttests revealed a correlation between frequency and receptive/productive gains. Mean scores showed that words mentioned with synonyms were learned most often, followed by task-essential words and last those mentioned without explanation. A two-way ANCOVA revealed main effects for cognates, and a statistical interaction between cognate status and types of exposure. A statistical correlation was found between receptive and productive gains. Aptitude scores correlated with productive gains but not with receptive gains. The results provide implications for ESL teachers who consider incidental learning of vocabulary within their conversation lessons.