Advances in Internet technology are making it possible to volunteer online through participation in research-based activities supporting non-profit and charitable organisations. Using survey data from a representative sample of contributors, this study investigates motivations to volunteer across a sample of five such online projects using the Volunteer Functions Inventory. We explore relationships between these motivations and actual recorded measures of both volunteer activity and retention. We also use quantile regression analysis to investigate the extent to which these motivations change at different stages in the volunteer process. Our results show that activity and retention tend to associate significantly and positively with the understanding and values motivations, as well as significantly and negatively with the social and career motivations. We also find the importance of motivations changes significantly among different percentiles of volunteer engagement. For some motivations, especially understanding, the nature of these changes is markedly different between activity and retention.