A systematic periodic DFT study of CH3OH oxidation on Pt(111) that involved the effect of solvation and sublayer Ru has been performed. Both the thermochemistry and the kinetics of all of the elementary reaction steps are addressed, and the reversible potentials of various elementary reaction steps during key intermediates CHO and CO formations are predicted; thus a detailed potential energy surface showing the interconversions of the various intermediates is presented. The effect of solvation and sublayer Ru on CH3OH oxidation mechanisms is revealed. Simultaneously, the origin of solvent effect on CH3OH oxidation mechanisms is also illustrated systematically through geometry configurations and electronic structure analysis, which can partially explain some experimental observations in UHV and electrochemical interfaces, especially the difference between the initial CH3OH oxidation pathways and the adsorbed residues that poisoned the pure Pt surface and impeded the performance of DMFCs. The present study ...