Cations are thermodynamically stable on certain transition metal surfaces, but so far no one has determined whether cations are catalytic intermediates or merely spectators. In this paper TPD and HREELS are used to determine whether methoxonium cations ([CH3OH2](+)((ad))) or closely related species are intermediates during the reaction H-2 + CH3OH --> CH4 + H2O on Pt(110). HREELS shows that the methoxonium cations are produced at the same temperature where reaction starts, and that the methoxonium cations decay in the same temperature range where the reaction stops, suggesting that methoxonium is a likely intermediate during the reaction. Replacement of CH3OH with CD3OH produces a 1.8-fold change in the yield of water and methane at 180 K due to a secondary kinetic isotope effect. The strong secondary kinetic isotope effect would be expected only if the reaction occurred via an S(N)1 pathway with a fully ionized transition state with the positive charge on the carbon. Together the HREELS and TPD results provide strong evidence that reaction goes via a carbocation pathway. with methoxonium cations or some closely related species as likely intermediates. This paper demonstrates for the first time that carbocations can be reactive intermediates during reactions on metal surfaces.