Abstract Water management is critical for PEMFCs, especially at system level. To study its impact on the performance and obtain useful indicators for fault detection, two commercial stacks were characterized by impedance spectroscopy under various humidities and current densities. One was fresh while the other was operated on-field during 10 000 h. Four capacitive and one inductive loops are identified. The capacitive loops are attributed to charge transfer kinetics at both anode and cathode and to cathodic and anodic mass transfer limitations. The inductive loop is attributed to oscillations in the ionomer water content induced by oscillations in the amount of produced water during the measurement. The size of the cathodic charge transfer loop and its departure angle at high frequencies increase as humidity decreases because the ionomer conductivity decreases. These parameters are potential indicators of the cathode drying. The angles are below 45° at low humidities but close to 90° in sur-saturated conditions.