A major barrier to the understanding of emotion dynamics in borderline personality disorder (BPD) lies in its substantial comorbidity with major depressive disorder (MDD) and bipolar disorder (BD). Whereas BPD has often been characterized in terms of dynamic emotional processes, including instability, reactivity, and inertia, its substantial comorbidity with MDD and BD makes it difficult to discern the specificity of these dynamics. To differentiate the emotion dynamics of BPD from those of MDD and BD, an experience sampling study of 38 participants with BPD, 15 participants with MDD, 14 participants with BD, and 62 healthy controls obtained reports of interpersonal challenges and emotions 5 times daily for 2 weeks. Interpersonal challenges included rejection, betrayal, abandonment, offense, disappointment, and self-image challenge; emotions included anger, excitement, guilt, happiness, irritability, and shame. Multilevel analyses revealed that heightened interpersonal reactivity of guilt and shame and heightened inertia of shame were relatively specific to BPD. These findings could not be accounted for by the presence of current MDD or BD. By contrast, heightened instability of anger and irritability and heightened inertia of irritability appeared to be largely transdiagnostic. Implications for clinical assessment, research, and theory are discussed.