Interfacial and perimeter sites have been known for their high activity in various reactions on supported gold nanoparticles. We find that the higher activity of interfacial sites in Au-13/TiO2(110) toward methanol decomposition originates from charge-transfer-induced Coulomb interaction among the gold, reactant, and reducible TiO2 support, brought about through the formation of an ionic O-Au bond between gold and methoxy in such sites, which turns the participating perimeter gold atom cationic. A direct result of such charge-transfer-induced repulsive interaction between cationic gold and positively charged C moiety of methoxy is activation of the positively charged C moiety of methoxy, as manifested by the pronounced elongation of O-C bond length and the tilting of the methoxy axis, which facilitate reaction of methoxy through C-H scission with the bridge oxygen atoms that are readily available from the reducible support. More generally, our proposed mechanism for the reactivity of the gold/TiO2 interface should hold for oxidation of organic molecules with the structure of R-O-R', where R and R' are (saturated) hydrocarbons.