Catalytic lean NO reduction system in periodic two steps, an operation in oxidizing conditions and a relatively short operation in reducing conditions, has been investigated using Rh-based catalysts that can hardly adsorb any nitrogen dioxide. The investigation was done using a pulse reaction with a pulse-injected reductant, a continuous pulse reaction in the periodic two steps, temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO) and transmission electron microscope (TEM). In the pulse reaction, after hydrogen or CO was pulse-injected for a moment to achieve reducing conditions, the highly dispersed Rh catalyst could catalyze NO reduction at 200-400 degreesC in oxidizing conditions for about 1 min. Furthermore, after the highly dispersed Rh supported on beta-zeolite was exposed to the gas composed of 3% of O-2, 40 ppm of SO2 and balance He at 300 and 400 degreesC for 24 h, NO was effectively reduced. The highly dispersed Rh supported on a beta-zeolite was reduced with hydrogen at a much lower temperature range and oxidized with oxygen at a much higher temperature range than Rh based catalysts with Rh particle size of several nm. (C) 2001 Elsevier Science B.V. All rights reserved.