The selective catalytic reduction (SCR) of nitric oxide by propane was investigated in the presence of oxygen over a H-ZSM5 catalyst. The concentrations of reactants were varied from 0% to 2% NO, 0% to 2% C3H8, and 0% to 35% O-2. Reactions were carried out at temperatures from 25 to 600 degrees C at a space velocity of 9600 h(-1). Periodic oscillations were observed in the NO concentrations of products near 400 degrees C when the feed contained a minimum of 2% NO, 1% C3H8, and 10% O-2. Random oscillations and chemiluminescent flashes were observed between 520 and 600 degrees C. The random oscillations occurred when the feedstock contained 2% NO, 2% C3H8, and 10-35% O-2, and the chemiluminescence occurred when the feedstock contained at least 2% NO, 1% C3H8, and 35% O-2. Results suggest that the SCR reaction proceeds via two different reaction pathways below and above 500 degrees C. The low-temperature reactions are possibly homogeneous-heterogeneous processes involving NO2 as the key reactant to form radical surface intermediates. At high temperatures, homogeneous radical reactions involving both NO and NO2 may be rate determining.