In order to estimate the effect of diffusion during the selective catalytic reduction of nitrogen oxides (NOx) using propane on Co-ion-exchanged ZSM-5, the catalytic activity was measured for the catalysts prepared from zeolites having different crystal sizes. The conversions of NOx and propane on a catalyst having a large crystal size (1.3 mu m) were much less than those on a catalyst having a smaller crystal size (0.10 mu m). Based on the experimental data and certain assumptions, the effective intracrystalline diffusion coefficient of NO during the reaction in the presence of water vapor at 673 K was estimated to be (6-9) x 10(-15) m(2)/s.