Access to the EU leads to a process of policy convergence in which member states’ institutions and policy cultures become increasingly adapted to align with an EU governance system. Especially in EU environmental policy, knowledge and expertise are key aspects of the institutions and policy cultures that are adapted in this process, which ideally results in the alignment of EU policy and administrative arrangements of member states. This paper offers a historical analysis of the Nitrates Directive's implementation in Poland and shows how increasing convergence of Polish institutions and cultures of expertise with EU policy occurred in response to the directive's requirements. The results highlight that (1) knowledge and expertise are central to policy convergence processes and that (2) institutions and cultures of expertise are entwined in ‘infrastructures of expertise’. The paper concludes with a call for more consideration of the science–policy interface in policy convergence processes related to Europeanisation.