The reaction of carbon monoxide with oxygen and with nitric oxide over silver and gold catalysts has been studied in a recirculating system with product removal. Nitrous oxide was the predominant nitrogen containing product of the latter reaction over both catalysts, nitrogen amounting to less than 2%. With gold the two reactions had similar characteristics both proceeding between -30 °C and 100 °C with identical pressure dependencies and activation energies close to zero. Oxidation by oxygen was about 40 times faster than that by nitric oxide and furthermore the latter reaction poisoned readily above 40 °C because of some interaction of nitric oxide with the surface. With silver, both reactions were first order in carbon monoxide pressure. For the CO + NO reaction the rate passed through a maximum as the NO pressure was increased eventually tending to inverse first order. The activation energy was 14 kcal mole-1 below 118 °C and 4 kcal mole-1 above that temperature. These results could be qualitatively explained by a Langmuir-Hinshelwood mechanism in which there is competition for a small number of sites. The CO + O2 reaction was zero order in oxygen and believed to proceed by a quite different mechanism.