Purpose: Despite the fact that prosocial motivation is related to word of mouth (WOM), few studies have been conducted to investigate the psychological and behavioral processes that mediate the two constructs. This study aims to explore customers’ relational interactions, specifically customer-to-employee interaction (via customer participation), customer-to-customer interaction and customer-to-brand interaction (via brand commitment), as mediators of the prosocial motivation–WOM linkage. Specifically, this paper examines the serial mediation model, in which prosocial motivation increases customer participation and customer-to-customer interaction, which in turn increase brand commitment and WOM sequentially. Design/methodology/approach: This study collected survey data from two different samples, including higher degree research education and fitness gym services (highly interactive, people-processing service contexts), and used partial least square method to analyze the multiple serial mediations. Findings: The results of this study show two serial mediating processes through which prosocial motivation influences WOM: 1. prosocial motivation → customer participation → brand commitment → WOM; and 2. prosocial motivation → customer-to-customer interaction → brand commitment → WOM. Practical implications: The findings provide managerial insights into how marketers can foster a more interactive service environment to encourage prosocial customers to engage in WOM more effectively. Originality/value: This study contributes to the literature on services WOM by illustrating the behavioral and psychological processes that underlie the effect of prosocial motivation on WOM.