One of the most relevant aspects of party democratization is the consequence of more inclusive methods of candidate selection, and particularly of primaries, on party competition. Despite its relevance, and contrary to the attention it has received in the US, the electoral consequences of primaries have rarely been analysed in Europe. This article assesses whether the use of one member one vote (OMOV) primaries for candidate selection affects the electoral result of the party and, if it does, in what direction. The article contributes to our still limited knowledge of this aspect of European party politics with an analysis of the use of closed party primaries by the Spanish Socialist Workers’ Party (PSOE) and their effects on its electoral performance in local elections. With an analysis of the electoral performance of the PSOE in municipalities of 10,000 inhabitants or more for the 1999, 2003, 2007 and 2011 local elections, the article shows that OMOV primaries have, overall, positive electoral payoffs but also that their effects can vary.