The electrocatalytic reactivity of Pt nanoparticles supported on high-pressure-high-temperature diamond particles towards adsorbed CO, methanol and formic acid oxidation is investigated employing differential electrochemical mass spectrometry (DEMS). Surface treatment of diamond particles, employed as dimensionally stable electrocatalyst supports, leads to materials with surfaces featuring mainly hydrogen (HDP) or oxygen-based functional groups (ODP). Pt nanoparticles with average diameter below 5 nm were generated by impregnation of the modified diamond particles. The voltammetric responses associated with the oxidation of adsorbed CO appeared unaffected by the surface termination of the diamond support. However, significant differences were observed for methanol oxidation in acid solutions, with Pt/HDP producing smaller current densities than Pt/ODP and a commercially available Pt catalyst (Pt/E-TEK). DEMS studies show higher conversion efficiencies to CO2 for Pt/ODP and Pt/E-TEK, while Pt/HDP exhibited values of approximately 90%. Evidence of formic acid generation as intermediate during methanol oxidation was obtained on all catalysts. Significant differences in the current density associated with the oxidation of formic acid were also observed, with Pt/HDP also providing the lowest current densities. The ensemble of the experimental data suggests that adsorbed HCOOads species is the key intermediate in methanol oxidation, and its subsequent oxidation to CO2 is strongly affected by the effective surface termination of the diamond support.