Electrochemical operation properties of SiO2-containing catalyst layers (SiO2-CLs) under low humidity were clarified experimentally in a polymer electrolyte fuel cell (PEFC). The mass activities at 0.85 V and current densities at 0.7 V increased with increasing SiO2 content below 53% RH at 80 degrees C. The use of SiO2-CLs led to decreased ohmic resistance due to increased water content. It was found that such an increase in water content in the SiO2-CLs increased the electrochemically active surface area (ECA) of the Pt catalyst, due to enhancement of the proton-conducting network in the electrolyte binder, and suppressed the specific adsorption of sulfonate anions on the Pt surface. It was also clarified that the SiO2 addition in the CL promoted the back-diffusion of water generated at the cathode to the anode, resulting in an increased O-2 gas diffusion rate to the cathode-CL/PEM interface, accompanied by an increased water content in the membrane. All of these factors greatly contributed to improve the cell performance under low humidity. (C) 2012 Elsevier Ltd. All rights reserved.