We have investigated cell voltage characteristics and AC impedance characteristics of polymer electrolyte fuel cells (PEFCs) at various humidity temperatures for H-2/O-2 and H-2/air test conditions (current density: 200. 400, and 600 mA cm(-2), cell temperature: 80 degrees C, humidity temperature at respective electrodes: 40, 50, 60, and 70 degrees C). The diffusion overpotential increases with decreasing humidity in the low humidity temperature region such as 40 and 50 degrees C and the Nyquist plots obtained from AC impedance measurements show a small arc superposed on an elliptic arc in the low frequency region. The diameter of this small arc increases with decreasing humidity temperature from 50 to 40 degrees C or with increasing current density. These results suggest that oxygen transport across the ionomer film in the catalyst layer is significantly reduced in the low humidity condition, which causes a decrease in cell voltage, increase in diffusion overpotential, the appearance of overlapping arcs (two separate arcs) in the lower frequency region on the Nyquist plots, and the increase of mass transport resistance from Nyquist plots. (c) 2008 Elsevier B.V. All rights reserved.