The oxygen reduction reaction (ORR) at the interface between platinum and Nafion 1100 equivalent weight was studied as a function of temperature (20-80 degrees C), humidity (10-100%), scan rate, the manner in which Nafion film was deposited, and the state of the Pt surface using ultramicroelectrodes employing cyclic voltammetry and chronoamperometry. ORR on smooth electrodes was strongly inhibited under specific conditions dependent on temperature, humidity, and scan rate. From the data presented, we postulate that dynamic changes in the molecular structure of the ionomer at the platinum interface result in differences in ORR voltammetry for films prepared and equilibrated under different conditions. The lack of similar changes for rough, platinized electrodes has been attributed to differences in initial ionomer structure and a higher energy barrier for ionomer restructuring. These model system studies yield insight into the ionomer-catalyst interface of particular interest for polymer electrolyte fuel cells. (C) The Author(s) 2015. Published by ECS. All rights reserved.