Online peer groups are a popular channel for mental health support, but the evidence for their effectiveness is mixed. The present study focused on empathy to better identify which supporters' comments regulated seekers' distress. We also explored how seekers' emotions may shape supporters' empathy. Posts (N = 7,646) published on an online peer support platform ("Emotional first aid [ERAN]") were sourced. Supporters' empathy (empathic concern, personal distress, exploration, and interpretation) and seekers' emotional expressions (soft negative, hard negative, and positive) were coded. We hypothesized that (1) empathic concern, exploration, and interpretation (but not personal distress) would predict better seekers' emotions (lower negative emotions and greater positive ones); (2) support seekers' soft negative and positive emotions would predict supporters' empathic concern and cognitive empathy (i.e., exploration and interpretation); but that (3) hard negative emotions would predict supporters' personal distress. A set of cumulative mixed models revealed that empathic concern predicted more seekers' positive emotions. However, cognitive empathy predicted more negative seekers' emotions. Seekers' soft negative emotions predicted greater expressions of supporters' empathy (of all types). Finally, seekers' positive emotions predicted more supporters' empathic concern and less personal distress, but also predicted less cognitive empathy (i.e., exploration). A secondary analysis found that this pattern of results differed to some extent as a function of the supporters' role as anonymous peers or the professional moderator. These findings suggest that empathy is a key component in online mental support platforms and that it may make online interactions more effective through emotional regulation. (PsycInfo Database Record (c) 2023 APA, all rights reserved).