In the selective catalytic reduction of NO by higher hydrocarbons, the influence of hydrocarbon molecular size on catalytic activity was investigated by using Cu-exchanged zeolites with different crystal sizes. The effect of zeolite structure was also investigated by using MFI- and MOR-type zeolites. In the case of n-octane as a reductant, the catalytic activity does not depend on the zeolite crystal size, indicating that the reaction is free from diffusional limitation. In the case of i-octane, on the other hand, the catalytic activity obviously depends on the crystal size of Cu-MFI zeolites, but not of Cu-MOR zeolites. The NO conversion over large crystallite Cu-MFI (1.29 mum) is less than that over small ones (0.10 mum). This crystal size effect was confirmed by using 2,2-dimethylbutane as a reductant. By measurements of transient response of in situ IR spectra, it was found that the reactivity of adsorbed NOx toward i-octane depended on the crystal size of Cu-MFI zeolite, but not for M-octane. These results clearly show that the observed crystal size effect is attributed to the smaller diffusivity of i-octane in Cu-MFI zeolite. It is concluded that the selective catalytic reduction (SCR) of NO by i-octane over Cu-MFI zeolite is restricted by the diffusivity of i-octane, which is determined by the molecular size of i-octane and the size of zeolite pore-opening, i.e. geometry-limited diffusion. (C) 2001 Elsevier Science B.V. All rights reserved.