The nonlinear frequency response behavior of a proton exchange membrane fuel cell (PEMFC) with different O-2 stoichiometry is characterized and analyzed by total harmonic distortion (THD) spectroscopy. Damjanovic oxygen reduction reaction (ORR) mechanisms with either oxygen electrochemisorption or oxygen chemisorption are comparatively studied in the simulation through THD spectroscopy and commonly used methods that include steady state current voltage (I-V) curve and electrochemical impedance spectroscopy (EIS). Comparison results show that only THD spectroscopy is able to make a fine distinction for the ORR mechanisms in certain kinetically controlled frequency ranges. The Damjanovic ORR mechanism with oxygen chemisorption is recognized as a better option for the ORR process in the PEMFC due to the reproducible experimental THD spectra. The observed nonlinear response in the frequency range from 2.5 Hz to 15.8 Hz is also recognized as the fingerprint of the ORR process. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.