Scanning electrochemical microscopy has been employed to spatially map the electrocatalytic activity of model proton exchange membrane fuel cell (PEMFC) catalyst films towards the hydrogen oxidation reaction (the PEMFC anode reaction). The model catalyst films mimicked the composition, structure and three-phase boundary conditions present in PEMFCs. The electrochemical characterisation was correlated with a detailed physical characterisation, achieved via dynamic light scattering, transmission electron microscopy and field-emission scanning electron microscopy. The platinum and carbon nanoparticle catalyst film constituents were found to be reasonably mono-dispersed, although the latter exhibited a strong tendency to agglomerate into porous bead-type structures when spun-cast. The number of carbon nanoparticles with little or no platinum was much more than would be expected based on the platinum-carbon mass ratio. Furthermore, the platinum-rich carbon particles tended to agglomerate and the clusters formed tended to be non-uniformly distributed. This morphology was reflected in a high degree of heterogeneity in the film activity towards the hydrogen oxidation reaction.