Anger and affective instability are key features of borderline personality disorder (BPD). Given the dynamic nature of affect, it is ideally studied using ambulatory assessment (AA). Recently, several major studies have examined affective instability via momentary self-report, using electronic diaries, which participants can use throughout their daily routine. The present study sought to complement this research by using an unobtrusive naturalistic observation method, the Electronically Activated Recorder (EAR). The EAR, which captures interpersonal behavior by periodically recording 50-second snippets of ambient sounds, was worn by 25 participants with BPD who also met the specific affective instability (AI) criterion as well as 13 participants with a depressive disorder (who did not meet criteria for AI or BPD) for three days. Trained coders listened to the captured recordings and rated participants' affect during each 50-second clip (i.e., in naturally varying social contexts). Results suggested that there were differences between diagnostic groups regarding the social context of anger, such that anger at a previous time interval predicted spending time alone in the subsequent time interval for the depressed group, but not for the BPD group. As an ambulatory observational method, the EAR offers an alternative to self-report and can provide insight into the naturalistic expression of emotions in BPD.