A comprehensive surface reaction mechanism on Pt is presented that is capable of describing CO oxidation, H-2 oxidation, water-gas shift (WGS), preferential oxidation (PROX) of CO, and the promoting role of H2O on CO oxidation reasonably well. This mechanism consists of a literature CO oxidation model, a surface reaction mechanism for H-2 oxidation on Pt developed here, and coupling reactions between the CO and H-2 chemistries included for the first time. Thermodynamic consistency, which is shown to be essential for WGS, is ensured in all steps of the entire mechanism. The CO-H-2 coupling via the CO + OH reaction, which may involve direct CO2 formation, CO* + OH* <----> CO2* + H*, as well as an indirect pathway via the carboxyl intermediate, is explored. It is shown that this coupling plays a significant role in capturing the promoting effect of H2O on the CO oxidation-temperature-programmed reaction experiments at low temperatures as well as the overall speed of the WGS and PROX reactions. With the parameters used here, the direct path dominates in the water-promoted low-temperature CO oxidation, whereas the indirect path is more or equally important in the WGS and PROX reactions, depending on the operating conditions. Finally, it is found that the facilitation of the disproportionation of H2O*, H2O* + O* --> 2OH*, via hydrogen bonding, is a possible mechanism for low-temperature CO oxidation via the direct path.