To determine the origin of the difference between reducing agents' reactivity for electroless deposition processes from the viewpoint of the molecular-level interaction at the solid-liquid interface, the mechanisms for the oxidation of formaldehyde and hypophosphite on Cu and Ni were analyzed and systematically compared by first principles density functional theory (DFT) calculations. The calculated energy diagrams for formaldehyde on Cu(111) showed that the pathway via intermediate CH2O22- was the most favorable. In this case, it was clear that water molecules had an effect. Here, water worked as a reaction mediator to remove the H atom from the OH group of formaldehyde, which resulted in the formation of the stable intermediate, CH2O22-. In contrast, in the case of hypophosphite, the oxidation on Cu(111) could not proceed, because there was no mediation effect from water on the hypophosphite reaction. On Ni(111), however, the oxidation of hypophosphite proceeded favorably, because the Ni surface could well stabilize the intermediate of the reaction due to its d-electron energy states. These results suggest that the mediation effect from the solvent and d-electrons of the metal surface are the key factors in determining the reaction mechanisms and reactivity of reducing agents on metal surfaces. (C) 2019 Elsevier Ltd. All rights reserved.