Surface-bound methoxy species are intermediates in a variety of surface processes, ranging from heterogeneous catalysis to advanced fuel cells, yet their chemistry on many metals remains elusive because of the difficulty of cleanly preparing adsorbed layers of these species. We propose that thermal dissociation of an adsorbed precursor, methyl nitrite (CH3ONO), can be used to produce methoxy species on reactive metal surfaces at low temperatures. On two Pt-Sn alloys, the methoxy intermediate is strongly stabilized (to 300 K) against thermal decomposition compared to Pt(111), where dissociation occurs at below 140 K, and there is a high selectivity to produce formaldehyde. These Pt-Sn alloys do not form the CO and H-2 dissociation products characteristic of methoxy chemistry on Pt(111).