This paper describes a detailed fundamental study regarding the influence of Cl species in γ-Cu(OH)₃Cl catalyzed oxidative carbonylation of methanol employing density functional theory (DFT) calculations as well as in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) experiments. The methanol was found positioned with the H atom towards the Cl atom—which in the first layer of the γ-Cu₂Cl(OH)₃(021) surface—in a close a-top position with a hydrogen bond between them at all adsorption sites; while the Cl atom plays a key role in the pre-reaction for H–O bond activation. The methoxide, which was formed through a substitution reaction, adsorbed on the surface through two O–Cu bonds; while the formed HCl weakly adsorbed on the surface and can easily escape from the surface. One new intermediate product was found during the calculation of minimum energy path (MEP), which connects the adsorbed methanol and coadsorbed methoxide and HCl, and its existence was confirmed through in situ DRIFTS experiments. During the reaction, the Cl atom escaped from the surface and bonded with the methanol first (forming CH₃OH⋯Cl) and then reacted with the methanol to form adsorbed methoxide and HCl. The existence of Cl seriously decreased the energy cost for methanol oxidation to methoxide.