This work demonstrates the impact of the nanostructure (pore size, wall thickness and wall crystallinity) of several carbon materials on their performance as oxygen reduction reaction (ORR) catalyst supports in PEM fuel cell applications. Two different mesoporous carbons vertical bar a surfactant-templated ordered mesoporous carbon (OMC) with 1.6 and 3.3 nm pores, and a silica colloid-imprinted carbon (CIC) with a 15 nm pore size], selected as being the most active in their class, were compared with microporous Vulcan Carbon. After loading with 20 and 40% Pt, both 3D transmission electron microscopy and electron tomography revealed that the Pt nanoparticles reside inside the majority of the pores of the OMC and CIC, but are located only on the outer surface of the VC particles. ORR performance studies on a rotating glassy carbon disc electrode in O(2)-saturated 0.5 M H(2)SO(4) showed that the Pt-loaded CIC outperforms both Pt-loaded OMC and VC. This is attributed to the higher electronic conductivity (due to the thicker and more crystalline walls, seen by both X-ray diffraction and thermogravimetric analysis) and facilitated mass transport in the larger pores of the CIC support. (C) 2011 Elsevier B.V. All rights reserved.