The Nafion-Pt interfaces in membrane electrode assemblies of operating fuel cells were studied by operando infrared spectroscopy. The potential dependence of atop adsorbed CO peak frequencies were measured over the potential range of 0-600 mV (vs. NHE) at 60 degrees C. Complex Stark tuning of peak frequencies arise from a combination of potential dependent coverage effects, and changes in the extent of back-donation from the metal d-band to the renormalized 2 pi* MO of COads. The Nafion-Pt interface was studied at higher potentials (initiating at open circuit) by examining platinum reflectivity as a function of electrode potential. The oxygen reduction onset-current is coincident with the observance of a 2% step-increase in Pt reflectivity and emergence of Nafion Pt interface spectra. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.