The characteristics of the impedance response of a Nafion 112 membrane in an operating polymer electrolyte fuel cell (PEFC) were investigated under subsaturated and fully humidified operation conditions using current step and ac impedance measurements in combination with high time resolution-high frequency resistance (HFR) measurements. The impedance as obtained from HFR measurements shows the behavior of a pure resistor only under fully humidified conditions. At low humidity, the impedance spectrum exhibits an inductive loop at low modulation frequencies below 1 Hz. The formation of this loop is associated with the slow uptake and release of water by the bulk of the solid polymer electrolyte with ac current. The results show that the appearance of a low-frequency inductive loop in the impedance spectrum of a low-humidity PEFC is essentially the result of this effect and, therefore, indicative of a limitation of cell performance by the hydration of the ionomer by the product water formed at the cathode. This effect must be taken into account when interpreting low-frequency inductive loops in PEFC impedance spectra, in addition to other explanations proposed in the literature, which have primarily focused on reaction steps in the oxygen reduction reaction and platinum oxidation/dissolution. (C) 2008 The Electrochemical Society.