Pulsed field gradient-nuclear magnetic resonance (PFG-NMR) has been applied to probe molecular motion of hydrocarbon probe molecules in the pores of deactivated catalysts. It is demonstrated that the technique works very well for coked catalysts, which were operated in an industrial reactor for up to 4 years. Cross-polarisation-magic angle spinning (CP-MAS) C-13 NMR was used to characterise the chemical nature of the coke deposited. It was found that reductions of up to 16% in average BJH pore diameter, 40% in surface area of 48% in pore volume of the catalyst occurred in the most heavily coked sample compared with the fresh catalyst. The decrease in pore volume could not be explained by the reduction in pore diameter alone, implying that significant pore blockage also occurred. The self-diffusivities of pentane and heptane probe molecules were found to decrease linearly with increasing coke content. The catalyst effectiveness factor was estimated to decrease by 10% in the most heavily coked sample compared with the fresh catalyst. This reduction in effectiveness factor was considered to have less influence on the overall rate of reaction per unit mass of catalyst than the considerable loss of pore volume. The tortuosities experienced by heptane imbibed in pellets were in the range 2.37-3.71, which was higher than those for pentane in the range 1.6-1.91; for both molecules, tortuosity showed a general increase with coke deposition. (C) 2003 Elsevier Science B.V. All rights reserved.