The influence of intracrystalline diffusivity on the selective catalytic reduction of NO with C3H8 over Co- and Cu-exchanged mordenites is examined by using mordenite samples of different crystal sizes. In the case of Co-mordenite, the intracrystalline diffusion limits the reaction and the Effectiveness factor decreases with decreasing reaction temperature contrary to the usual case. In the case of Cu-mordenite, the rate of NO reduction does not depend on the crystal size of mordenite in spite of a larger reaction rate than that over Co-mordenite, which indicates that the intracrystalline diffusion in Cu-mordenite is much faster than that in Co-mordenite. These results are discussed in terms of adsorption-controlled diffusion: That is, strong adsorptive interaction of NO with Co ion, examined through TPD of NO, should be responsible for the slower diffusion rate in Co-mordenite and for the unusual temperature dependence of effectiveness factor of Co-mordenite.