This work investigated coke induced physical changes in pore geometry and intraparticle diffusivities of deactivated unimodal and bimodal catalysts withdrawn from a 4 ton/day direct coal liquefaction ebullated bed reactor. Porosimetry, surface area, equilibrium adsorption, and pore diffusivity measurements were performed on fresh, spent, and regenerated catalysts. Equilibrium adsorption experiments showed the loss in adsorption sites in the spent catalysts to be significantly greater than indicated by the decrease in overall BET surface areas. Pore diffusivity measurements performed using solute adsorption from a finite bath surrounding the catalyst particles yielded pore diffusivities two orders of magnitude lower than would be expected based on the measured average pore characteristics of the spent catalysts. This observation was postulated to result from a nonuniform radial distribution of coke deposits within the spent catalyst particles. A mathematical diffusion model incorporating the nonuniform coke deposition aspect of the deactivation process was formulated and used to simulate experimental diffusional uptake experiments. The model yielded results in satisfactory agreement with experimental measurements and provided a reasonable rationalization for the low diffusivities in the spent catalysts using physically realistic parameter values.