Crowdsourcing in recent times has become popular among not-for-profits as a means of eliciting members of the public to contribute to activities that would normally have been carried out by staff or by contracting external expertise. The GLAM (galleries, libraries, archives, museums) sector does have a history of involving online volunteers (e.g. reviewing books). Extending that tradition, some GLAM institutions are engaging in crowdsourcing projects to enhance and enrich their collections. But what motivates the public to participate in these crowdsourcing activities? Understanding the unique motivations of participants is needed to establish a motivational framework for GLAM organisations in their not-for-profit context. We present findings from a study of the motivational factors affecting participation in the Australian Newspapers Digitisation Program (ANDP) by the National Library of Australia (NLA). Based on motivational theories and frameworks the study shows that the participants are motivated by a complex framework of personal, collective and external factors. Participants were highly intrinsically motivated, but valued altruistic and community motivations as well. Community and external factors played a vital role in their continued involvement. The paper concludes with a conceptual framework of the motivational factors for crowdsourcing participants in a GLAM context based on the motivational dynamics observed in the ANDP case.