This study examined the temporal stability of employee preferences for rewards over seven monthly evaluations. Participants completed a ranking stimulus preference assessment monthly, and the latter six monthly assessments were compared to the initial assessment. Correlations of preferences from month to month ranged from r = -0.89 to 0.99. Contrary to the stationarity axiom of rational choice economic theory, but consistent with a behavioral economic perspective, preferences for rewards changed across time for all participants and sometimes even reversed for some. This temporal instability suggests that organizations implementing incentive programs using preference assessments should periodically re-evaluate employee reward preferences and not assume temporal constancy of preferences. (Contains 1 figure.)