The effect of intracrystalline, diffusion on the selective catalytic reduction of NO with hydrocarbons was investigated by using cation-exchanged zeolite catalysts of different crystal sizes. In the reaction with n-alkane over Cu-MFI and Cu-MOR, the reaction rate did not depend on the zeolite crystal size. In the case of multi-branched alkanes, on the other hand, the observed reaction rate over Cu-MFI was slower over larger crystal zeolite, but, over Cu-MOR, the rate did not depend on the crystal size. Thus, it was revealed that the reaction with larger hydrocarbons was restricted by geometry-limited diffusion depending on the hydrocarbon molecule size and zeolite pore size. In the reaction with ethene and propene over Cu-MFI, the rate evidently depended on the zeolite crystal size, indicating that the reaction is controlled by intracrystalline diffusion in spite of small size of hydrocarbon molecules. Moreover, contrary to the usual cases, the catalyst effectiveness factor increased with the reaction temperature. Similar results were obtained in the reaction even with propane over Co-MOR. These results led us to propose a concept of adsorption-controlled diffusion: strong adsorptive interactions of alkene with CU2+ and NO with Co2+ should be responsible for the slower diffusion of alkene in Cu-MFI and NO in Co-MOR and for the unusual temperature dependence of the effectiveness factor. (C) 2004 Elsevier B.V. All rights reserved.