The effect of periodic operation over Pt and Pt/Ba catalysts supported on gamma-alumina under oxidizing conditions was investigated using simulated automotive exhaust gas from lean-bum combustion. The conversion of hydrocarbons and NOx was measured in cycled feedstream and steady feedstream under oxidizing conditions. The activities of the catalysts were improved in the cycled feedstream between oxidizing and reducing atmospheres under average oxidizing conditions. In particular, the NOx reduction on the Pt/Bt catalyst was higher than that on the Pt catalyst in the cycling operation. From the mass spectral analysis in streams of NO-O-2-C3H6 and NO-O-2-H-2 (balance He) gas, it was found that NO was oxidized and stored on the Pt/Ba catalyst under oxidizing conditions, and that the stored NOx on the catalyst was subsequently reduced to N-2 under reducing conditions.