The combination of hydrogen and oxygen on the surface of a palladium catalyst at low partial pressures at the temperature of the laboratory is completely inhibited by very small amounts of carbon monoxide. The poisoning effect is only temporary and, during the induction periods, which are observed, the carbon monoxide is removed by the oxygen in the mixture of hydrogen and oxygen. After the induction period combination proceeds at the normal rate. The length of the inert periods observed increases with the amount of carbon monoxide initially added to the system, and the rate of removal of the carbon monoxide increases as its pressure decreases, and becomes comparatively very great when the carbon monoxide present does not exceed the amount required to cover the surface of the metal. The observations recorded can be explained by a development of the hypothesis of De la Rive that the catalysis of the reaction between hydrogen and oxygen, and between carbon monoxide and oxygen, involves the alternate oxidation of the metal and the reduction of the surface oxide by hydrogen or by carbon monoxide respectively.