This paper tests the effect of a matching mechanism on donations in a controlled fieldnexperiment. We match the donations of students at the University of Zurich who, each semester, have to decide whether they wish to contribute to two Social Funds. Our results support thenhypothesis that a matching mechanism increases contributions to a public good. However, the effect depends on the extent to which the contributions are matched. Whereas a 25 percentnincrease of a donation does not increase the willingness to contribute, a 50 percent increase does have an effect. In addition, people need to be socially inclined to react to the matching mechanism. The field experiment provides some evidence suggesting that the matching mechanism crowds-out the intrinsic motivation of giving.