A sulfide antioxidant buffer solution was used to trap and concentrate trace quantities of H2S that permeated through 50 cm(2) samples of fuel cell Nafion 117, 212, and 112 membranes using a H2S partial pressure difference up to 1000 ppm at room temperature. Experiments were conducted between 24 and 48 h to achieve sulfide ion concentrations high enough to be determined accurately by subsequent titration with Pb(NO3)(2). In contrast to previous work, a special H2S mixing system was employed to achieve reproducible delivery of H2S concentrations throughout each experiment. The calculated rates of H2S crossover, normalized for membrane area, thickness, and differential PH2S varied from as low as 7.58 x 10(-10) to 4.65 x 10(-9) g/s atm cm depending on the type of Nafion sample and humidity conditions. Even low levels of humidification of the membrane and gases (saturated at 25 degrees C) increased the crossover rate of H2S in the Nafion materials. H2S permeation through the electrolyte membrane negatively impacts the oxygen reduction rate occurring on the cathode and possibly causes an irreversible decrease in the overall fuel cell performance. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3258285] All rights reserved.