Nafion/TiO2 composite membranes with different TiO2 contents were studied in an H-2/O-2 proton exchange membrane fuel cell (PEMFC) over a wide range of relative humidity (RH) values from 26 to 100% at temperatures of 80 and 120° C. The composite membranes, which were prepared using a recast procedure, showed a pronounced improvement over unmodified Nafion membranes when operated at 120° C and reduced RH. For instance, at 50% RH, the Nafion/20% TiO2 membrane demonstrated a performance identical to that of an unmodified Nafion membrane operated at 100% RH. This performance level was comparable to that of a bare Nafion membrane at 80° C. The high performance of the Nafion/TiO2 composite membranes at low RH was attributed to improved water retention due to the presence of absorbed water species in the electrical double layer on the TiO2 surface. The zeta potential and thickness of the hydrodynamically immobile water layer at the TiO2/water interface were discussed as parameters influencing the water balance in the membranes. The obtained experimental PEMFC performance data were fitted using an analytical equation, and calculated parameters were analyzed as functions of RH and TiO2 content in the composite membranes. © 2005 The Electrochemical Society.