Bridging social dominance theory and labour studies, this field study investigated the mechanisms underpinning the relationship between rejection of group-based domination and participation in union activities. Respondents (N = 135) were members of a public sector union in California, that is, a hierarchy-attenuating institution. Results revealed that union identification mediated the negative relationship between social dominance orientation and active union participation. Moreover, the mediational effect of union identification was moderated by perceived union instrumentality (i.e. outcome-and process-based benefits afforded by the union), indicating that the relationship between union identification and participation was stronger among those union members who consider that the union affects workplace justice. The findings reveal the importance of both identity-based and instrumental motivations underlying union participation. The novelty of applying social dominance theory to union behaviour is underscored.In the United States, the labour movement has suffered an important decline in the last 25 years: unions lost approximately 5.2 million members in both private and public sectors with membership rates sinking to 12.5% in 200412.5% in (Chaison, 2006. Despite declining participation, the public still thinks labour unions are necessary to protect the working person (Panagopoulos & Francia, 2008). As active members mainly contribute to achieving the goals of unions, it is crucial for the survival and vitality of unions to understand the motivations driving union members to participate in formal (e.g. participation in union meetings and rallies) and informal (e.g. helping another member to know about the union) union activities. Understanding motivations to participate in union activities can also uncover