Background <p>Regardless of the communication channel used, intimate conversations foster interpersonal emotion regulation (IER). Although private chatting with friends is important for adolescents, research concerning online IER remains limited.</p> Methods <p>Cross-lagged panel models were used to study the directionality of offline and online co-rumination, co-dampening, and co-enhancing in 449 adolescents across a one-year period (53% boys; <i>M</i>_age = 15.25 at T1). Additionally, depressive symptoms, friendship quality and the frequency of online interactions were taken into account.</p> Results <p>A unidirectional association from offline to online co-rumination was established, with adolescents’ tendency to co-ruminate in-person transferring to later private chat conversations. Bidirectional associations were found for co-dampening and co-enhancing. Adolescents who were inclined to use online co-dampening respectively co-enhancing during online conversations also showed an increased tendency for offline co-dampening respectively co-enhancing and vice versa. Regarding the interrelation with depression, more severely depressed adolescents used more offline co-rumination one year later. Advantageously, offline co-enhancing created a buffer against these symptoms. Lastly, both a “dose–response” and a “response–dose” relationship were discovered for co-rumination. Accordingly, the more online private interactions adolescents had, the more they used this communication style during chat conversations with their momentary best friend one year later. Relatedly, the more offline co-rumination adolescents used, the less they turned to the online modality to interact with others. No prospective relationships were identified for friendship quality.</p> Conclusion <p>The study sheds light on which IER-modality should be primarily addressed during mental health support.</p>