We explore the implementation of the European Water Framework Directive from the point of view of institutional economics, focusing on participation in the introduction of management plans. The design of participatory processes is linked with decision-making and implementation costs through an analytical model. The model is subsequently applied to qualitative evidence from four different empirical cases. The analysis reveals similarities between widely heterogeneous cases: process design choices seem generally to economize on decision-making costs. The paper discusses the implications of these findings for policy implementation and future research. Copyright © 2013 John Wiley & Sons, Ltd and ERP Environment.