Nafion 112 membranes were soaked in I M H2SO4 solutions containing variable amounts of Fe and Cr ions, either individually or mixed. An even distribution of the metal ions on the surface of the membranes was observed with electron probe microanalysis (EPMA) mapping. The proton conductivity of the soaked membranes was investigated using a conductivity cell. For Fe ions, the conductivity was almost constant until the Fe-ion solution concentration reached 300 ppm. Over the 300-ppm threshold, the conductivity decreased significantly. Similar results were obtained with Cr ions in the membrane, but here the threshold was approximately 200 ppm in the solution. Mixed metal ions were found to decrease these threshold values due to the additive effect of the two metals. (C) 2008 Published by Elsevier B.V.