Improving emotion regulation is central to borderline personality disorder (BPD) treatment, but little research indicates which emotion regulation strategies are optimally effective and when. Basic emotion science suggests that engagement emotion regulation strategies that process emotional content become less effective as emotional intensity increases, whereas disengagement strategies that disengage from it do not. This study examined whether emotional reactivity to emotional stimuli predicts the effectiveness of engagement and disengagement emotion regulation across self-report, general physiologic (heart rate), sympathetic (skin conductance responses), (...) and parasympathetic (respiratory sinus arrythmia) emotion in BPD, healthy, and clinical control (i.e. generalized anxiety disorder; GAD) groups. 120 participants (40 per group) were exposed to emotion inductions and then instructed to implement engagement (mindful awareness) and disengagement (distraction) strategies while self-report and physiological emotion measurements were taken. In the BPD and GAD groups, higher heart rate or respiratory sinus arrythmia reactivity, respectively, predicted improved mindful awareness effectiveness. Higher skin conductance reactivity predicted worsened distraction effectiveness in BPD. Higher reactivity may potentiate engagement emotion regulation, and exacerbate disengagement from emotional content, in BPD. Future research should examine other domains of emotion regulation that may be influenced by emotional intensity, and other forms of emotional intensity that may influence them. (shrink)