Background: Differentiation of the bipolar disorders (BP) from a borderline personality disorder (BPD) can be challenging owing to shared features, with emotional dysregulation being the likely principal one. Aim: To assess differences in emotion regulation strategies in those with BP alone, BPD alone and those comorbid for both. Methods: We interviewed participants previously receiving a BP or BPD diagnosis, studying those who met DSM criteria for one or both conditions. Results: The sample comprised 83 with bipolar disorder, 53 with BPD and 54 comorbid for both. Analyses established linear trends, with the greatest impairment in emotion regulation strategies in the comorbid group followed by the BPD group, and with the lowest in the BP group. Specific deficits in the comorbid group included impulsivity, difficulties with goal directed behaviour, and accessing strategies. A similar linear profile was quantified for maladaptive cognitive emotion regulation strategies, weighted to catastrophizing and rumination. Adaptive emotion regulation strategies were superior in the bipolar group, without significant differences observed between the comorbid and BPD groups. Limitations: Reliance on self-report measures; combined BP I and II participants limits generalisability of results to each bipolar sub-type; use of DSM diagnoses risking artefactual comorbidity; while there was an over-representation of females in all groups. Conclusions: Differences in emotion regulation strategies advance differentiation of those with either BP or BPD, while we identify the specificity of differing strategies to each condition and their synergic effect in those comorbid for both conditions. Study findings should assist the development and application of targeted strategies for those with either or both conditions. Crown Copyright (C) 2016 Published by Elsevier B.V. All rights reserved.