Background and AimsCannabis and alcohol are the most commonly used (il)licit drugs world-wide. We compared the effects of cannabis and alcohol use on within-person changes in impulsivity, hostility and positive affect at the momentary and daily levels, as they occurred in daily life. DesignObservational study involving ecological momentary assessments collected via electronic diaries six random times a day for 28 consecutive days. SettingOut-patients' everyday life contexts in Columbia, MO, USA. ParticipantsNinety-three adult psychiatric out-patients (85% female; mean=30.9years old) with borderline personality or depressive disorders, who reported using only cannabis (n=3), only alcohol (n=58) or both (n=32) at least once during the study period. MeasurementsReal-time, standard self-report measures of impulsivity, hostility and positive affect, as impacted by momentary reports of cannabis and alcohol use. FindingsCannabis use was associated with elevated feelings of impulsivity at the day level [b=0.83, 95% confidence interval (CI)=0.17-1.49] and increased hostility at the momentary (b=0.07, 95% CI=0.01-0.12) and person (b=0.81, 95% CI=0.15-1.47) level. Alcohol use was associated with elevated feelings of impulsivity at the momentary (b=0.42, 95% CI=0.13-0.71) and day levels (b=0.82, 95% CI=0.22-1.41) and increased positive affect at the momentary (b=0.12, 95% CI=0.06-0.18) and day (b=0.33, 95% CI=0.16-0.49) levels. ConclusionsCannabis and alcohol use are associated with increases in impulsivity (both), hostility (cannabis) and positive affect (alcohol) in daily life, and these effects are part of separate processes that operate on different time-scales (i.e. momentary versus daily).