We examined the psychometric properties of an experience-sampling measure of affect (PANAS) using data from self- and peer reports. A multivariate multilevel model was used to assess the reliability of the latent PANAS scales at the within- and between-person level. Findings suggest satisfying internal consistencies for self- and peer reports of affective experiences at both levels of analysis. Convergent and discriminant validity of the two affect scales were examined by means of a multilevel multitrait-multimethod approach (MLM-MTMM) indicating distinct findings at the within- and between-person level. These findings provide further insights into the structural relations between the two PANAS scales: Whereas positive and negative affect were unrelated at the between-person level; they were negatively correlated at the within-person level. Copyright © 2010 John Wiley & Sons, Ltd.