Abstract: Pt catalysts with different average particle sizes (1.7, 3.0, and 5.0 nm) were supported on Vulcan XC-72 carbon black through ethylene glycol reduction of H₂PtCl₆. Using transmission electron microscopy, we found a geometric interaction between the micropores on the surface of carbon black support and the supported Pt catalysts. Small Pt nanoparticles (with an average particle size of 1.7 nm) were found to be frequently incorporated into the micropores, and thus appeared to be covered by a thin carbon layer and embedded in the carbon matrix. However, such a phenomenon was absent for Pt particles with larger average particle sizes (3.0 and 5.0 nm), which generally showed an exposed clean surface. The different interaction with the micropores therefore induced an abnormal particle size effect of the supported Pt catalysts on their electrochemically active surface area, and had an influence on their mass electrocatalytic activity towards methanol oxidation.