Journal article: Why do public buyers use competitive bidding instead of negotiation to select a contractor? To shed light on this question, we consider the impact on the main economic determinants identified in the literature on the choice of auctions versus negotiations, namely competitive intensity, buyer's budgetary constraints and level of complexity of the project to be done. We then examine a comprehensive data set of public sector building contracts awarded in France in 2007. The analysis suggests that the choice of awarding procedures can not be fully explained by economic efficiency considerations. Alternative explanation involving political aspects may usefully be considered since public buyers are elected and therefore aware of the political impact of their decisions.