The present work aims to show the nonlinear behavior of a PEM fuel cell under different mass transport conditions. Understanding this behavior helps in online state-of-health monitoring and control of a fuel cell stack. To analyze the health of an operating stack, a total harmonic distortion analysis (THDA) system requires only the sum of voltages or currents of the stack to be monitored. A low-frequency current or voltage signal is impressed on the fuel cell stack, and the resulting voltage or current signal is measured. To determine any change in harmonics, the measured signal is processed with a harmonic analyzer. The operational states of individual cells of the fuel cell stack may be inferred from at least one change in the harmonic content of the impressed signal. The mass transport problem related to the cathode and anode is distinguished using mixed-frequency signals. The present study found experimentally that hydrogen starvation is dominantly observed in the harmonic analysis only below a frequency of 15 Hz, whereas air starvation showed harmonic changes at frequencies below 100 Hz. Total harmonic distortions were observed to rise to 2-2.5% under both the starvation conditions but with different frequency signals. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.