The effects of cathode microporous layer (MPL) composition including carbon black and polytetrafluoroethylene (PTFE) loadings on the performance of a proton exchange membrane fuel cell (PEMFC) are evaluated under different relative humidity (RH) at air inlet. Both acetylene black and composite carbon black which is made of acetylene black and black pearls 2000 (BP) are used in the experiments. The results show that the optimal cell performance at different air RH depends on the cathode MPL composition. When acetylene black is used, the optimal composition at higher RH is found to be 1.5 mg cm(-2) carbon loading and 20 wt% PTFE content, while an increase in PTFE content could improve the cell performance at lower RH states. The cell performance also could be enhanced significantly at lower RH when composite carbon black is employed, while it fails to increase the cell performance at higher RH. The composite carbon black with 30 wt% BP is found to give the optimal cell performance at low RH conditions. The results benefit the understanding for the role of cathode MPL in running PEM fuel cells under insufficient air humidification. (C) 2013 Elsevier B.V. All rights reserved.