A measurement-model-based error structure analysis was applied to impedance data collected under galvanostatic modulation for a single 5 cm(2) polymer electrolyte membrane (PEM) fuel cell. The stochastic errors were found to be roughly independent of frequency and were similar for the two instruments used. The measurement-model approach was found to be capable of identifying both high-frequency and low-frequency artifacts in the spectra. The low-frequency inductive loops were found, in some cases, to be consistent with the Kramer-Kronig relations. In other cases, nonstationary phenomena were found to have influenced the low-frequency response. This work showed that once steady-state operation was achieved, the low-frequency inductive loops in the impedance response could be associated with physical phenomena within the fuel cell. In addition, the formalism of the measurement-model error analysis provides a means for determining whether a steady state has been achieved. (c) 2007 The Electrochemical Society.