An organizational field study examined the mechanisms through which supervisorsÕ interactional justice influences employeesÕ individual-directed organizational citizenship behaviors (OCBs). Although, previous research supports the relationship between interactional fairness and OCBs, the mechanisms of operation remain unclear. Separate frameworks, based on role enlargement and role discretion exist, with no prior attempts to compare them. In this study, I examine two competitive models, (a) a role enlargement model, using employeesÕ role definitions as a mediator and (b) a role discretion model, using employeesÕ role definitions as a moderator. The results, based on data collected from 141 employee-manager dyads in a work organization, confirm that role definitions moderate the relationships between interactional justice and OCBs. The findings are discussed to derive theoretical and practical implications and directions for future research.