Drawing on cognitive rumination theories and conceptualizing customer service interaction as a goal attainment situation for service employees, the current study examined employee rumination about negative service encounters as an intermediate cognitive process that explains the within-person fluctuations in negative emotional reactions resulting from customer mistreatment. Multilevel analyses of 149 call-center employees’ 1,189 daily surveys revealed that on days that a service employee received more (vs. less) customer mistreatment, he or she ruminated more (vs. less) at night about negative encounters with customers, which in turn led to higher (vs. lower) levels of negative mood experienced in the next morning. In addition, service rule commitment and perceived organizational support moderated the within-person effect of customer mistreatment on rumination, such that this effect was stronger among those who had higher (vs. lower) levels of service rule commitment but weaker among those who had higher (vs. lower) levels of perceived organizational support. Theoretical and practical implications of these findings are discussed.(PsycINFO Database Record (c) 2016 APA, all rights reserved)