This paper reports findings from social science research carried out in Kambia, Northern Sierra Leone during first year of an Ebola vaccine trial (August 2015–July 2016). The study used a range of qualitative methods to explore participant motivations for volunteering for the study. The authors analyse participants’ perceptions of the risks and benefits of participations, highlighting the importance of a contextual approach, focusing on four types of motivation: altruism; curiosity and hope; health-seeking; and notions of exchange. They argue for the role of social science in developing grounded research ethics and community engagement strategies that can take into account context and local realities.