We wish to propose a neutron reflectivity (NR) experiment for a Nafion/platinum interface under controlled electrochemical potential, because NR is the only technique to study buried interfaces non-destructively in ambient atmosphere. Platinum and Nafion are catalyst and ionomer of a cathode layer for polymer electrolyte fuel cells (PEFCs). At a platinum/Nafion interface oxygen reduction reaction (ORR) occurs and dominates on the cathode performance. Since voltage loss during ORR in the cathode is an essential problem for development of PEFCs, we should improve the interfacial structures by analyzing the interface at nanometer scale to realize eco-friendly FC vehicles.