NOx reduction in a different reaction mechanism from the NOx storage reduction was investigated. NOx is found to be reduced over a specified catalyst in the following steps: in lean conditions, NO is partially oxidized into NO2, followed by NO and NO2 adsorption, in rich conditions, the adsorbed NOx is reduced mainly into ammonia, followed by ammonia adsorption and finally, during the subsequent cycle of the lean conditions, the adsorbed ammonia selectively reacts with gaseous NOx. In this reaction, nitrogen is mainly not formed in the rich operation, but in the lean operation. On the other hand, nitrogen is formed only in the rich operation over the NOx storage-reduction catalyst. The newly developed catalyst is composed of a noble metal/an oxygen storage capacity (OSC) material and a solid acid. (C) 2007 Elsevier B.V. All rights reserved.