The influence of intracrystalline diffusion on the selective catalytic reduction of NO by hydrocarbons (propane. ethene) was investigated by using Cu-MFI catalysts of different zeolite crystal sizes. The influence of hydrocarbon as a reductant on the diffusion process was also investigated, employing kinetic and temperature-programmed desorption studies. In the NO-C3H8-O-2 reaction, the apparent reaction rate of NO conversion into Nz did not depend on the zeolite crystal size, which indicates that the reaction is not controlled by intracrystalline diffusion. In the NO-C2H4-O-2 reaction, on the other hand, the apparent reaction rate evidently depended on the zeolite crystal size; the reaction rate over large crystal Cu-MFI (1.29 mu m) was significantly less than that over a small one (0.09 mu m), which indicates that the reaction is controlled by intracrystalline diffusion. The larger diffusion resistance in the NO-C2H3-O-2 reaction was attributed to the slower diffusion rate of ethene in zeolite channels than propane, which was due to much stronger interaction of ethene with Cu-MFI catalyst. Thus, the adsorption property of hydrocarbon on the Cu-MFI catalyst is revealed to play an important role in determining intracrystalline diffusivity and the diffusion influence on the selective reduction of NO over zeolite catalysts. (C) 2000 Elsevier Science B.V. All rights reserved.