This study examined incidental receptive and productive vocabulary gains within conversation class interactions. Sixteen Mexican learners of English attended four videotaped conversation lessons where 40 target words were incorporated into 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 of exposure and receptive/productive gains. Mean scores showed that students most often learned task-essential words, followed by words mentioned with synonyms, and last, those mentioned without an explanation. A two-way ANCOVA revealed main effects for cognates, and a statistical interaction between cognate status and types of exposure to target words, and a moderate effect of frequency of mention on receptive knowledge. Results provide implications for ESL teachers who consider incidental learning of vocabulary within their conversation lessons.