Abstract The establishment of European Regulatory Networks (ERNs) has been presented as a way to promote the uniform implementation of EU law in a range of issue areas. This article argues that the level of co-ordination achieved in ERNs crucially depends on the level of interdependence between Member State authorities in a given issue area. Drawing on a survey among national representatives in two ERNs and a set of in-depth interviews, it shows that both the uniformity of implementation and the use of common purpose as a way to reach co-ordination are strongly associated with the perceived level of interdependence.