Assessment of internalizing symptoms has generally relied on cross-sectional and retrospective self-reports, but ecological momentary assessment (EMA) is increasingly used to capture quick fluctuations in symptoms, enhance ecological validity, and improve recall accuracy. However, there are very few measures of internalizing symptoms that have been validated for use in EMA designs. In Study 1, we chose candidate items for EMA short forms of the Dysphoria and Well-Being scales from the Inventory of Depression and Anxiety Symptoms (IDAS), based on principal factor analyses and internal consistency analyses conducted on aggregated cross-sectional datasets (total N = 8,876). In Study 2, we tested the items using an EMA design in a sample of college students (N = 279) oversampled for elevated neuroticism. Scale structure, reliability, and convergent and discriminant validity (regarding baseline IDAS scales, baseline affect, and EMA affect) were evaluated at the within- and between-person levels using multilevel structural equation modeling. Exploratory and confirmatory factor analyses in separate subsamples revealed the expected two-factor structure, yielding a four-item Well-Being scale and a five-item Dysphoria scale. Both scales showed acceptable to good internal consistency, strong convergent validity, and generally adequate discriminant validity. However, some associations of the new scales with EMA affect (i.e., Dysphoria with negative affect; Well-Being with positive affect) were very strong at the between-person level, such that they were not empirically distinct. Overall, this study provides an initial validation of brief EMA-IDAS Dysphoria and Well-Being scales that can be used in research or clinical settings, with particular utility for capturing within-person, dynamic effects. (PsycInfo Database Record (c) 2022 APA, all rights reserved).