A Nafion thin film on a platinum surface is a model for a reaction center in a catalyst layer of a polymer electrolyte fuel cell (PEFC). The cross-sectional structure of a Nafion film in a humidity chamber is unveiled by neutron reflectometry using a contrast variation method. A condensed layer of Nafion molecules approximately 3nm in thickness is found on the platinum surface, where a depletion layer of water is formed. Because a similar dense layer of Nafion was observed at the Nafion/platinum interface when the Nafion surface was in contact with bulk water in a trough, the Nafion molecules should be tightly bound to the platinum surface. This segregated layer of the composition would be an activation barrier to oxygen diffusion from air to the platinum catalyst in a PEFC cathode.