Research that has sought to understand why employees engage in organizational citizenship behaviors (OCB) has concentrated on between-person variables, typically ignoring intraindividual influences. Accordingly, we know much about who engages in OCB, in general, but know relatively little regarding under what circumstances people engage in OCB. By integrating social comparison with affective events and just-world theories, we propose and test a dynamic model wherein directional social comparisons are expected to have direct (automatic-motivational) and indirect (affective) intraindividual effects on OCB. The hypotheses were tested using multilevel modeling on 1076 observations from 99 participants that were collected via an interval-contingent experience sampling methodology. The results provide support for the hypotheses that social comparisons are related to OCB through positive affect and the direct effects of social comparisons on OCB are moderated by beliefs in a just world. Theoretical and practical implications are discussed. Copyright © 2011 John Wiley & Sons, Ltd.