We reexamined the psychometric properties of the Momentary Impulsivity Scale (MIS) in two young adult samples using daily diary (N = 77) and ecological momentary assessment (N = 147). A one-factor between- and within-person structure was supported, though “I felt impatient” loaded poorly within-person. MIS scores consistently related to emotion-driven trait impulsivity; however, MSSDs of MIS scores were unrelated to outcomes after accounting for aggregate MIS scores. We observed positive, within-person correlations with negative, but not positive, affect. Between-person MIS scores correlated with alcohol problems, though within-person MIS-alcohol relations were inconsistent. MIS scores were unrelated to laboratory-based impulsivity tasks. Findings inform the assessment of state-level impulsivity in young adults. Future research should prioritize expanding the MIS to capture the potential multidimensionality of state-level impulsivity.