Corrosion of the three carbon black materials which differ in their textural, sub-structural, and morphological characteristics is studied in H2SO4 electrolyte at 80 A degrees C at positive polarization. Impregnation of carbon blacks with Nafion(A (R)) ionomer is found to strongly affect their susceptibility to corrosion, which varies non-monotonically as a function of the Nafion(A (R)) loading. Low temperature nitrogen adsorption is applied to study the distribution of the ionomer molecules in the carbon matrix.