This study used ecological momentary assessment (EMA) to measure positive affect (PA) and negative affect (NA) among people who inject drugs (PWID) and examined associations with borderline personality disorder (BPD) symptoms and difficulties with emotion regulation, in the context of injection drug use. We recruited PWID, ages 18-35 years, through syringe exchange program sites in Chicago, Illinois. After completing a baseline interview including a screener for BPD and the Difficulties in Emotion Regulation Scale (DERS), participants used a mobile phone app to report mood, substance use, and injection behavior for 2 weeks. Participants who completed at least 2 EMA assessments were included in the analysis ( = 161). The mean age was 30, about 1/3 were women, 63% were non-Hispanic White, and 23% were Hispanic. In multivariable mixed-effects regression models, positive BPD screen was associated with greater momentary NA intensity and greater instability of both NA and PA. Independent of BPD screening status, DERS score was associated positively with momentary NA intensity and instability and negatively with PA intensity. This finding suggests that emotion dysregulation is an appropriate target for assessment and intervention. Whereas concurrent withdrawal was associated with both greater NA and less PA, opioid intoxication was associated with only greater PA. We did not find support for our hypothesis that emotion dysregulation would moderate the effect of withdrawal on NA. Findings support the validity of the EMA mood measure and the utility of studying mood and behavior among PWID using EMA on mobile phones. (PsycInfo Database Record (c) 2020 APA, all rights reserved).