The oxygen reduction kinetics in a Nafion-impregnated gas diffusion electrode was investigated as a function of the Nafion content under the mixed migration and charge transfer control, using electrochemical impedance spectroscopy (EIS) and a potentiostatic current transient (PCT) technique. For this purpose, the gas diffusion electrodes were prepared with different Nafion contents in order to control the Nafion length distribution across the active catalyst layer (ACL). From the comparison of the nitrogen gas adsorption isotherms of Nafion-impregnated Pt/C powder specimens with the isotherm of pure Pt/C powder specimen, it was found that as the Nafion content within the ACL increased, the average Nafion length increased, and the Nafion length distribution became narrower. From the analysis of the impedance spectra considering the Nafion length distribution, it was confirmed that as the Nafion content increased, the charge transfer resistance R(ct) was reduced slowly, but the component diffusivity of proton D(H)(+) was enhanced rapidly. From the analysis of the cathodic PCTs in consideration of the Nafion length distribution, it was concluded that when the Nafion content increased, oxygen reduction was dominantly controlled by charge transfer rather than by proton migration under the mixed migration and charge transfer-controlled constraint. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.2987944] All rights reserved.