The electrocatalytic oxidation of formic acid (HCOOH) and formate (HCOO-) to CO2 on platinum has been studied over a wide range of pH (0-12) by surface-enhanced infrared absorption spectroscopy (SEIRAS) coupled with cyclic voltammetry. The peak current of HCOOH/HCOO- oxidation exhibits a volcano-shaped pH dependence peaked at a pH close to the pK(a) of HCOOH (3.75). The experimental result is reasonably explained by a simple kinetic model that HCOO- oxidation is the dominant reaction route over the whole pH range. HCOOH is oxidized after being converted to HCOO- via the acid-base equilibrium. The ascending part of the volcano plot at pH < 4 is ascribed mostly to the increase of the molar ratio of HCOO-, while the descending part at pH > 4 is ascribed to the suppression of HCOO- oxidation by adsorbed OH or oxidation of the electrode surface. In acidic media, HCOOH is adsorbed on the electrode as formate with a bridge-bonded configuration. The bridge-bonded adsorbed formate is stable and suppresses HCOO- oxidation by blocking active site. However, the suppression is not fatal because bridge-bonded adsorbed formate enhances the oxidation of HCOO- at high potential by suppressing the adsorption of OH or surface oxidation. The complex cyclic voltammograms for HCOOH/HCOO- oxidation also can be well interpreted in terms of the simple kinetic model. The experimental results presented here serve as a generic example illustrating the importance of pH variations in catalytic proton-coupled electron transfer reactions. (C) 2014 Elsevier Ltd. All rights reserved.