This study examines the effects of operating parameters-comprising temperature, relative humidity, hydrogen pressure, and membrane thickness-on hydrogen crossover rate in a polymer electrolyte membrane fuel cell (PEMFC). It is found that the hydrogen crossover rate increases proportional to both temperature and relative humidity for all membrane samples. Increased hydrogen crossover rate is also observed with increasing hydrogen pressure. The hydrogen crossover rate increases gradually with the decrease of membrane thickness from 258 to 135 gm. When the membrane thickness decreases from 63 to 21 gm, there is a dramatic increase of hydrogen crossover. Multiple linear regression analysis was used to analyze the effects of all the operating parameters on hydrogen crossover rate. The results indicate that increased hydrogen crossover rate is mainly determined by the inverse of the logarithmic membrane thickness, followed by hydrogen pressure, relative humidity, and temperature, respectively. (C) 2013 Elsevier Ltd. All rights reserved.