The effect of the addition of Pr on the nature of the lattice oxygen of CeO2 was investigated. In order to enhance the mobility of the lattice oxygen for monitoring, Rh (2 wt.%) was supported on the Pr/Ce composite oxides which were calcined at various temperatures. The decomposition of N2O was employed as a test reaction. Although an addition of Pr to CeO2 had only a small effect when the composite oxides were calcined at low temperature (600 degrees C), it had a large accelerating effect for N2O decomposition when the calcination temperature was increased. Especially the performance of the Ph-supported composite oxide with 20 mol% of Pr calcined at 800 degrees C was higher than that of Rh/CeO2 in which CeO2 was calcined at 600 degrees C. Oxygen desorbed more easily from the catalysts containing Pr than from Rh/CeO2. Thus, the interaction of Pr with CeO2 increased the mobility of the oxygen of the latter, leading to the high activity of the catalysts to decompose N2O.