This study examined the contribution of the psychological contract framework to understanding organizational citizenship behavior (OCB) using survey data gathered at three measurement points over a three year period from 480 public sector employees. Separating perceived contract breach into its two components, the data suggest that perceived employer obligations explained unique variance in three dimensions of citizenship behavior (helping, advocacy and functional participation) beyond that accounted for by perceived employer inducements. Employees’ acceptance of the norm of reciprocity moderated the relationship between employer inducements and the dimensions of advocacy and functional participation. Employees’ trust in their employer moderated the relationship between perceived employer obligations and the dimensions of advocacy and functional participation. Contrary to the hypothesis, procedural or interactional justice were not found to moderate the relationship between the psychological contract and OCB. The implications of the findings for psychological contract research are discussed.