This study investigates whether the altruism and courtesy dimensions of organizational citizenship behaviors (OCB) regulate mood at work. Social psychological theories of mood regulation suggest helping behaviors can improve individuals' moods because helping others provides gratification and directs attention away from one's negative mood. We capture mood states prior to and following the enactment of OCBs using experience sampling methodology in a sample of managerial and professional employees over a 3-week period. Results suggest altruism shows a pattern consistent with mood regulation; negative moods during the prior time period are associated with altruism and positive moods in the subsequent time period. The pattern of results for courtesy behaviors is only partially consistent with a mood regulation explanation. Consistent with theories of behavioral concordance, interaction results suggest individuals higher on Extroversion have more intense positive mood reactions after engaging in altruistic behaviors. Interactions with courtesy were not significant.