The effects of the porosity and pore size distribution of catalyst carbon support on the proton conduction and electrode performance have been studied by comparing two carbon supports [Vulcan XC 72 (V) and Ketjenblack (KB)]. Differences in the H-2/O-2 cell performance of the two catalysts are shown to be consistent with differences in the electrodes' proton resistivity and the area of the Pt catalysts available for the oxygen reduction reaction (ORR) at a similar area specific activity. For the same ionomer-to-carbon weight ratio (I/C) and relative humidity (RH), the electrode with KB-supported catalyst exhibits higher proton resistivity than the one with Vulcan-supported catalyst. The cumulative surface area and cumulative volume distributions of the two catalysts were obtained from N-2 BET measurements. It is concluded that the difference in surface area and pore volume for these two catalysts comes from pores smaller than 4 nm. The extent of ionomer captured within these small pores is substantial for Pt/KB and minor for Pt/V. It is shown that the ionomer captured within these small pores of the primary carbon particles does not contribute to the proton conductivity across the electrode. If the total electrode I/C-ratio is corrected for the amount of ionomer captured within the primary carbon particles, the dependence of the proton resistivity of Pt/KB and Pt/V on the effective I/C-ratio becomes identical. Since Pt/KB contains more pores smaller than 4 nm, higher total I/C-ratio is needed for Pt/KB to achieve the same proton resistivity as Pt/V based electrodes. Pt particles are more uniformly dispersed on higher surface-area carbon supports, resulting in a higher Pt area available to ORR. The selection of the proper carbon support depends on the porosity and its effect on cathode proton conduction, ORR kinetics and electrode durability. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3562945] All rights reserved.