While there is strong evidence for the psychometric reliability of the Positive and Negative Affect Schedule-Expanded Form (PANAS-X) in cross-sectional studies, the between- and within-person psychometric performance of the PANAS-X in an intensive longitudinal framework is less understood. As affect is thought to be dynamic and responsive to context, this study investigated the multilevel reliability of PANAS-X Positive Affect, Negative Affect, Fear, Sadness, and Hostility scales. Generalizability theory and structural equation modeling techniques (coefficient ω) were employed in four ecological momentary assessment samples (N = 309; 41,261 reports). Results demonstrate that the PANAS-X scales, including short versions of the Positive and Negative Affect scales, can reliably detect between-person differences. PANAS-X scales also were able to reliably measure within-person change, though these estimates may be impacted by scale content and study design. These results support the use of the PANAS-X in daily life research to intensively measure affect in the natural environment. (PsycInfo Database Record (c) 2023 APA, all rights reserved).