The impact of party primaries on turnout at the meso level has been particularly under-researched. Thus, the aim of this paper is to explore the determinants of turnout on party primaries by employing an original dataset comprising over 300 selection processes of candidates and leaders in Spain from 1991 to 2021 at both the national and regional level. One of our main findings suggests, in line with what has been affirmed by previous literature, a positive impact of competitiveness on turnout. We also find some contradictory effects of ideology on turnout: as expected most left-wing parties have better turnout averages than right-wing parties, but turnout is higher in primaries organised by right-wing parties. In addition, the type of primaries and the (online-offline) format of the event also appear to be a relevant factor shaping participation.