C-13 solid-state NMR spectroscopy has been used to study the various species formed during the oxidation of ethylene on Ag catalysts. The NMR spectra obtained after heat treatments of sealed catalyst samples provide direct observation of surface species which can be identified by their chemical shift values. This study also reports C-13 shifts for standard compounds dispersed on Ag/eta-Al2O3 and Ag/SiO2 which provide groups that can be present under reaction conditions, and the decomposition of C-13-labeled ethylene oxide on Ag/SiO2 in the absence of O-2 was investigated. On the basis of these experiments, C-13 assignments have been made for the following adsorbed species : ethylene oxide, formic acid, formate, oxalic acid, oxalate, acetic anhydride, acetaldehyde, and carbonate. In the absence of O-2, ethylene oxide decomposed to acetic acid, acetaldehyde, and formaldehyde, and broad peaks indicative of polymers on the SiO2 surface were also observed. The observation of ethylene oxide adsorbed on Ag/SiO2 after reaction between (C2H4)-C-13 and O-2 was consistent with a previous study showing this catalyst was much more selective than a Ag/eta-Al2O3 catalyst; CO2, adsorbed (C2H4)-C-13, and a deprotonated oxalate or formate species were also identified. The formation of CO2 in the absence of aldehyde or acetate species but in the presence of oxalate species suggests an alternative route may exist for complete oxidation that does not involve isomerization of ethylene oxide.