The effect of various oxygen passivation treatments on a 15 wt% MoP/SiO2 and a 15 wt% Ni2P/SiO2 catalyst is reported. Approximately 85 % of the MoP was re-oxidized when the MoP/SiO2 catalyst was passivated or simply exposed to air, whereas only about 40 % of the Ni2P was re-oxidized when the Ni2P/SiO2 catalyst was treated similarly. A second reduction of the catalysts after the passivation, followed by CO uptake measurements, showed an increase in CO uptake in the case of the MoP/SiO2 and a significant loss in CO uptake on the Ni2P/SiO2 when compared to the CO uptake measurements of the corresponding non-passivated samples. The oxidized species that resulted from the passivation of the Ni2P/SiO2 were much more easily reduced than those on the MoP/SiO2. The loss in CO uptake observed in the case of the Ni2P/SiO2 is attributed to sintering and/or restructuring of the Ni2P during the second reduction step. [GRAPHICS] .