In this paper we study the determinants of volunteering. We depart from previous literature that considers only economic incentives and investigate the role that extrinsic and intrinsic motivations play in shaping the supply of voluntary labour. We build on previous theoretical research and derive a framework for interpreting results from our econometric analysis. We consider a sample of Italian workers of which we can observe proxies of motivations and volunteering choices in three different sectors: social services, political activism and union activism. We make due allowance for the endogeneity of motivations and show that both types of motivations have an effect on volunteering. Consistently with theoretical models, we find extrinsic motivations to lower voluntary labour supply, whereas the opposite holds for intrinsic motivations. Copyright CIRIEC, 2004.