Catalytic lean NO reduction system in periodic two steps, lean/rich operations has been investigated over Rh-based catalysts. The investigation was done using a pulse reaction. In the reaction, after H-2 or CO was pulse-injected for a moment to achieve reducing conditions, the highly dispersed Rh catalyst could catalyze NO reduction at 200-400degreesC in lean conditions for 1 min. Furthermore, NO was effectively reduced over the highly dispersed Rh/beta-zeolite after exposure to the gas composed of 3% of O-2, 40 ppm of SO2 and balance of He at 300 and 400degreesC for 24 h.