Elementary schoolchildren's working memory performance (WMP) fluctuates from moment to moment and day to day, yet the underlying mechanisms are not well understood. In the present study, affective states were investigated as predictors of these fluctuations. Interindividual differences in the intraindividual affect-WMP associations were expected, and their structure was explored. One hundred nine children (8-11 years) were investigated in an ambulatory assessment. Affective states (positive affect [PA], negative affect [NA], activation, deactivation) and WMP were assessed 3 times daily for up to 31 consecutive days. In the whole sample, WMP was lower at occasions with higher NA or deactivation, while there was no overall effect of PA or activation. Results of a mixture model analysis revealed meaningful heterogeneity in these effects: Approximately half of the children showed comparably weaker effects of affect on WMP, while the other three groups showed (1) comparably stronger negative effects of NA and deactivation; (2) a comparably stronger positive effect of activation; or (3) comparably stronger negative effects of NA and deactivation and stronger positive effects of PA and activation. Findings emphasize the importance of explicitly considering interindividual differences in intraindividual associations. They are discussed in the context of current frameworks of interindividual differences in environmental sensitivity. (PsycINFO Database Record (c) 2019 APA, all rights reserved).