Co-rumination is a dyadic process between relationship partners that refers to excessively discussing problems, rehashing problems, speculating about problems, mutual encouragement of problem talk, and dwelling on negative affect. Although studies have addressed youths' "tendency" to co-ruminate, little is known about the nature of co-ruminative conversations. The primary goal of the present study (N = 314 adolescent friend dyads) was to identify microsocial processes that sustain and reinforce problem talk among adolescent co-ruminating friends. Results indicated that co-rumination was characterized by friends responding to each other's statements about problems with engaged statements (e.g., questions, supportive statements) that elicited even more problem talk. Results also indicated that some aspects of co-rumination (i.e., extensively talking about problems, rehashing problems, speculating about problems, and mutual encouragement of problem talk) were associated with positive friendship adjustment, whereas other aspects (i.e., dwelling on negative affect) were associated with internalizing problems. The present research highlights the utility of attending to microsocial processes in friends' conversations and has implications for intervention.