The influence of the ionomer type in DMFC anode catalyst layers (ACLs) on the properties of primary and secondary pores was investigated. Two hydrocarbon ionomers, Fumion (R) SO-340 and Fumion (R) S-360, were compared with Nafion (R). Electrochemical properties of MEAs and ACES were determined by galvanostatic U/i-characteristics, methanol stripping and impedance spectroscopy. Structural properties and chemical compositions of the ACES were determined by SEM/EDS and automated standard porosimetry. Substitution of Nafion (R) by hydrocarbon ionomers causes a decrease of maximum power density and active surface area by up to 48% and 91%. Ionomer phases formed by Fumion (R) ionomers have a mean tortuosity twice as high as Nafion (R) ionomer phases. The relative percentage of primary pore volume decreases in the order: Nafion (R) > Fumion (R) SO-340 > Fumion (R) S-360, indicating an increased blocking of primary pores by Fumion (R) ionomer films. The proton conductivities of the primary pores are a factor of 2-5 higher than those of the secondary pores. The results are explained by an enhanced film-forming ability of Fumion (R) ionomers compared to that of Nafion (R), leading to a higher tortuosity of the ionomer phase. By encapsulating primary pores, Fumion (R) ionomer films block active sites and reduce the active surface and the performance of ACES and MEAs. (C) 2012 Elsevier B.V. All rights reserved.