Whereas some studies have shown that contingently applied extrinsic rewards decrease intrinsic motivation created by an interesting task, an equal number of studies have failed to support this phenomenon, known as the overjustification effect. Research studies from work and organizational psychology journals were categorized according to whether intrinsic motivation had been measured via free-time or task performance measures. Results of a meta-analysis, testing for a moderator effect, show that support for the overjustification effect occurs only when intrinsic motivation is operationalized as task behaviour during a free-time measure. In contrast, task performance measures indicate that the effects of intrinsic and extrinsic rewards on motivation are additive. The advantages and disadvantages of different operationalizations of the intrinsic motivation construct are discussed.