This paper builds an experiment to investigate whether asking people to state how much they will donate to a charity (to pledge) can increase their actual donation. Individuals' endowment is either certain or a random variable. We study different types of pledges, namely private, public and irrevocable ones, which differ in individual cost of not keeping a promise. Public pledges appear to be associated to lower donation levels. Irrevocable pledges ensure an amount of donations equal to donations in absence of pledges. Moreover, a significant number of individuals keep their promises, in presence of either private or public pledges. A higher risk attached to the endowment increases donations.