The electrocatalytic behavior of boron-doped nanoporous honeycomb diamond films modified with Pt nanoparticles (10–150 nm) was examined with cyclic voltammetry (CV) and electrochemical impedance spectroscopy in acid solution. For two such films as well as an as-deposited film, the average number of exposed surface Pt atoms was estimated from CV to be 1.07(±0.05)×1015 cm−2 (real area, as estimated by SEM). These electrodes exhibited high electroactivity for hydrogen adsorption and oxidation of several alcohols. The current density (geometric basis) in the CV for methanol oxidation at a Pt-modified porous film with a pore diameter of 400 nm and a pore depth of 3 μm, was greatly enhanced, by a factor of 16, in comparison to the values obtained with a bulk Pt electrode. This enhancement is attributed to both the high surface area of the nano-honeycomb structure and the high electrocatalytic activity of Pt nanoparticles dispersed inside the pores. The electrocatalytic activities of the Pt-modified nano-honeycomb films were found to be dependent on the structural parameters of the honeycomb pores and the molecular sizes of the alcohols. Interestingly, ac impedance measurements have indicated a decrease in the penetration depths for a Pt-modified porous film with a pore diameter of 60 nm with increasing alcohol size, and an increase in the reaction resistances for ethanol oxidation with decreasing pore diameter.