We present density functional calculations for the adsorption of hydroxymethylidyne (COH) on Pt, PtRu, and PtRuMo (111) surfaces. Here we clarify the adsorption mechanism by using a charge transfer analysis related to the adsorption energy. We observe that the preferred binding sites for COH are the hcp hollow Pt–Pt–Pt, hcp hollow Pt–Ru–Pt, and hcp hollow Pt–Ru–Pt adsorption sites for Pt, PtRu, and PtRuMo, respectively. Addition of Ru to form a PtRu surface increases the adsorption energy, while addition of Mo to form a PtRuMo surface decreases it. Our analyses show that the adsorption energy is determined by electron transfer between the molecular COH and the metal surfaces associated with bonding.