Ceria-supported Pt and Pd catalysts have been synthesized by the combustion method. The catalysts are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Pt and Pd metals are ionically dispersed on the CeO2 surface of crystallite sizes in the range of 15-20 nm. In 1% Pt/CeO2 Pt is found to be in the +2 and +4 oxidation states whereas Pd is in the +2 state in 1% Pd/CeO2. Catalytic activities for NO reduction by CO, NH3, CH4, and C3H8 and CO, CH4, and C3H8 oxidation by O-2 have been investigated over all these catalysts using the temperature-programmed reaction technique. The results are compared with Pt and Pd metals dispersed on alpha -Al2O3 support prepared by the combustion technique. Both oxidation and reduction reactions occur at much lower temperatures over M/CeO2 compared to those over M/Al2O3 (M = Pt, Pd). The rate and turnover frequency of NO + CO and CO + O-2 reactions over M/CeO2 are higher than those over M/Al2O3. The observed enhanced catalytic activity of M/CeO2 has been attributed to the ionic dispersion of Pt and Pd on CeO2 leading to a strong metal-ceria interaction in the form of solid solution, Ce1-xMxO2-(4-n)x/2, having linkages of the type -O2--Ce4+-O2--Mn+-O2--(n=2, 4) on the CeO2 surface. (C) 2000 Academic Press.