Abstract The oxidation of Rh field-emitter surfaces during the CO oxidation reaction has been investigated by imaging atom-probe mass spectroscopy. Rhodium samples were heated to 500 K in various mixtures of O 2 and CO at total pressures between 1 and 2 Torr (1 Torr = 133.3 N m −2) and were subsequently transferred under ultrahigh vacuum to the atom probe for analysis. Surface oxides were detected when the O 2 CO ratio was 40 1 or greater, but not when the ratio was 30 1 or less. The 40 1 ratio correlates with the ratio where deactivation of the reaction has been observed, and the results provide direct evidence that oxide formation is the surface process responsible for deactivation. Quantitative atom-probe analysis indicated that the oxide formed during the reaction is stoichiometric Rh 2O 3.