Pair dynamics has received much attention in collaborative writing (CW) research. Prior research on the link between pair dynamics and L2 learning opportunities (often operationalized as language related episodes; LREs) in CW has primarily focused on the quantity, length, and outcome of LREs, but rarely considered the quality of the LREs. Also, scarce research has considered the fluctuating nature of pair dynamics and its impact on the L2 learning opportunities. This study employs an approach of detecting pair dynamics that accounts for its fluid nature to investigate how learners' collaboration in various aspects of CW may influence the quantity, quality, focus, and outcome of the LREs in CW tasks by intermediate L2 learners. Results indicated that while discussion on task management and organization generally took up a limited portion of dyadic interaction, unbalanced contribution to these aspects, especially when coupled with an unequal involvement in content elicitation, greatly affected the quantity and quality of L2 learning opportunities in CW. Also, pairs who participated unequally in discussing language use might still generate many LREs. However, a moderate portion of these LREs represented one learner's externalized private speech, and thus might not be very beneficial for the other pair member.