This article examines the ‘demand’ side of the market for political activism and analyses the extent to which grassroots activists of political parties and politically oriented civil society organizations (CSOs) resemble or differ. With data from several surveys of CSO and Green party activists in Spain we evaluate how much the pool of members from all these organizations are part of what can be identified as a single market of activists. After describing the socio-economic, political socialization and recruitment backgrounds, as well as the internal and external engagement of activists, we conclude that – while sharing some common features – activists of CSOs and Green parties are less alike than frequently assumed and that the analogy of a single market of political activism may be more misleading than helpful in our understanding of the challenges that parties face to counteract declining membership trends.