Investigators theorize that those with Borderline Personality Disorder (BPD) have a distinct style of relating to others typified by interpersonal hypersensitivity. How this hypersensitivity manifests in close relationships remains unclear. The current project seeks to refine this characterization of the interpersonal dysfunction domain in BPD through examination of romantic partner conflict and trait rejection sensitivity (RS). In this study, two groups of participants (20 BP trait +, 38 BPD trait -) and their romantic partners (total N=116) carried an electronic diary for one week and reported on daily mood and interpersonal interactions. I found no statistically significant difference in trait RS between B+ participants and B- participants. BPD features measured dimensionally were modestly correlated with RS, however. RS was positively associated with a higher proportion of negative behaviors (e.g. throwing things) to positive behaviors during conflict for individuals in the B+ group, while individuals in the B- group demonstrated a negative association between rejection sensitivity and negative conflict behaviors. Positive behaviors were not associated with trait RS. Partner RS was associated with fewer negative behaviors in actors. Concerns that the romantic partner would end the relationship were associated with both actor and partner reports of conflict, but not with trait RS. Momentary-reported conflict-related negative cognitions were not well-explained by RS or group status. Negative affect and behaviors are perhaps best studied ecologically and interpersonally (e.g., within dyads) if we are to better understand and dysfunctional social processes and personality pathology.