We investigate the mechanistic origin of oxidation of HCOO- on Pd/C in alkaline media using H/D kinetic isotope effect studies and electrochemical experiments, such as cyclic voltammetry, step-linear sweep voltammetry, and stripping voltammetry. We reveal that HCOO- is oxidized to CO2 through a direct pathway, and the oxidation of H-ads is the rate determining step. The slow oxidation kinetics of H-ads, which causes a site-occupying effect, suppress the oxidation of HCOO- , and a more distinct phenomenon is observed in the potential region for H-ads. These observations provide clues to understanding the significant electrocatalytic activity of Pd and rational approaches for the development of more active catalysts for the oxidation of HCOO- in alkaline media. (c) The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected].