We have studied the effect of hydrothermal aging on the oxygen storage/release and activity in a commercial automotive catalyst containing Pt, Rh, Ni, and Ce. X-ray photoelectron spectroscopy (XPS) was used to measure changes in the oxidation state of the Ce and Ni present in the washcoat of the catalyst after various oxidation and reduction treatments and rate measurements of the carbon monoxide oxidation reaction were used to determine the reaction kinetics and activity. The catalyst was tested fresh and after hydrothermal aging at 1000 degrees C under cyclic redox conditions. To determine which Ce species were present on the surface, we fit the XPS Ce(3d) data with combinations of spectra from standard materials (CeO2 and Ce2O3). Based on our measurements we conclude that aging the catalyst causes a loss of oxygen storage capacity due to sintering of the ceria particles which reduces the ceria/noble metal interaction and does not allow the Ce to cycle between oxidation states. In addition, hydrothermal aging causes the loss of oxygen storage of the Ni due to the irreversible formation of NiAl2O4. For the fresh catalyst, the observed kinetics for the uncycled carbon monoxide oxidation reaction show all of the signatures attributable to a catalyst with a high degree of ceria/noble metal interaction (complete suppression of the carbon monoxide inhibition effect, decreased sensitivity of the reaction rate to gas-phase oxygen concentration, and decreased apparent activation energy). Changes in the kinetics upon aging are consistent with a loss of ceria/noble metal contact area due to sintering of the ceria particles.