This paper investigates the effects of gas crossover. Specifically, mathematical simulations are conducted to elucidate the fundamental changes in fuel cell operation as permeation of the various gases through the membrane increases. Two cases are explored, with the first one examining uniform increases in the set of gas-permeation coefficients and the second one the existence of regions of high gas crossover (i.e., membrane pinholes). For the first case, operation at 120 degrees C is studied and a maximum limit for the hydrogen permeation coefficient of 1x10(-10) mol/bar cm s for a 25 mu m membrane is determined. For the second case, it is shown that negative current densities and temperature spikes can arise due to mixed-potential and direct-combustion effects where there are large enough pinholes, thereby impacting performance and water and thermal management. (C) 2008 The Electrochemical Society.