This study investigated the nature of dyadic interaction in an adult ESL classroom. The study was longitudinal, classroom based, and examined the nature of interaction between 10 pairs of adult ESL students over a range of language tasks and over time (a semester). Four distinct patterns of dyadic interaction were found. These patterns are distinguishable in terms of equality and mutuality (Damon & Phelps, 1989). More importantly, the findings suggest that certain patterns of dyadic interaction are more conducive than others to language learning. These findings are explained by reference to Vygotsky’s theory of cognitive development.