Motivation Crowding Theory (Frey and Jegen, 2001) suggests that, under certain conditions, monetary incentives may have a negative impact on consumer willingness to engage in word of mouth (WOM) (H1), by crowding out the intrinsic motivation towards this behavior (H2). Three experiments tested these predictions in the context of fictitious WOM marketing (WOMM) campaigns designed to promote a preferred commercial brand, institution or political candidate. It was also hypothesized that the incentives would have a negative impact on the quality of the task performance (H3). Data shows support for all three hypotheses in the case of the commercial brand, but not when participants recommended a preferred institution or political candidate. Results are discussed in view of relevant accounts of Motivation Crowding Theory in economics and psychology: the overjustification hypothesis, the signaling hypothesis, and Cognitive Evaluation Theory. Possible moderators of the crowding-out effect are suggested, and the theoretical implications of using Motivation Crowding to advance advertising literature and inform WOM marketing practice are discussed.