Receptive syringe sharing (RSS) among people who inject drugs (PWID) is a risk factor for hepatitis C virus and HIV infections. PWID with borderline personality disorder (BPD) have increased risk of RSS, but it remains unclear what drives this association. This study used ecological momentary assessment (EMA) to study characteristics associated with BPD, and RSS among PWID. We recruited PWID, ages 18-35, through two Community Outreach Intervention Projects syringe service program sites in Chicago, Illinois. After a baseline interview, participants used a mobile phone app to respond to momentary surveys on mood, substance use, and injection risk behavior for two weeks. For each momentary assessment, ratings on negative mood descriptors were combined to create measures of total negative affect (NA), and NA components of dejection, shame, anger, irritability, and worry. RSS was defined by participant responses indicating that they had used a syringe that someone else had used. We estimated mixed effects logistic models, regressing RSS on baseline affect-related impulsivity, lagged momentary NA, and the interaction term. Out of 163 participants who completed at least two EMA assessments, 152 (93%) reported at least one injection event and had valid pre-injection mood assessments required to be included in the analysis. We found that affect-related impulsivity, combined with worried mood in the hours preceding the injection episode, predicted increased risk of RSS. PWID having difficulties with emotion regulation may be at increased risk of RSS during periods of anxiety or tension. (PsycInfo Database Record (c) 2020 APA, all rights reserved).