In a previous paper, (Chen, N.; Blowers, P.; Masel, R. I. Surf. Sci. 1998, 418, 329-341.) we found that methoxonium ions (CH3OH2)(+) can form when methanol and hydrogen coadsorb on Pt(110). In this paper, we characterize the chemistry of methoxonium on (2 x I)Pt(110) using mainly TPD. We observe three reaction pathways: a simple dehydrogenation to CO and hydrogen; an S(N)1 pathway to form water, methane, and adsorbed CH. groups; and an S(N)2 pathway yielding dimethyl ether. The S(N)1 pathway shows a strong secondary isotope effect implying that the transition state is ionic. The S(N)2 pathway is seen at high hydrogen coverages and shows kinetics typical for an S(N)2 process. This work demonstrates for the first time that cationic species on surfaces react via the same S(N)1 and S(N)2 pathways seen during reactions of cations in solution.