Although pair work is advocated by major theories of second language (L2) learning and research findings suggest that pair work facilitates L2 learning, what is unclear is how to best pair students in L2 classes of mixed L2 proficiency. This study investigated the nature of pair work in an English as a Foreign Language (EFL) class in a college in Saudi Arabia. The L2 proficiency of the learners in such classes is often quite heterogeneous. Thirty learners allocated into similar (high-high and low-low) and mixed-L2 proficiency pairs (five pairs in each proficiency pairing) completed a short composition. The audio recorded and transcribed pair talk was analysed for the learners' overt focus on language use and amount of L2 used. In our analysis we took into consideration the effect of proficiency pairing as well as the dyadic relationship the learners formed. Our findings suggest that decisions regarding how to best pair students in heterogeneous classes depend on the aim of the activity, and that the dyadic relationship may be of greater significance than proficiency pairing. (Contains 7 tables.)