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A technique has been developed for measuring the hydrogenation activities of hydrodesulfurization catalysts before, during, and after the liquefaction of coal. Using this technique, rapid and severe deactivation of catalysts occurs within the first few hours of on-stream coal processing. This is a nonpermanent type clearly caused by the deposition of carbonaceous materials in the pore structure of the catalysts. Catalysts can be fully regenerated after short processing times to nearly their virgin active states. After their initial period of deactivation, the catalysts attain a level of activity that strongly depends upon their chemical composition, surface area, and the distribution of surface area with the pore structure. Catalysts with a preponderance of surface area in pores with r > 6.0 nm show a higher level of activity. The early stage of deactivation is related to carbon laydown and not the ash content of the coal; however, coals with high hydrogen contents are slightly less prone to this type of aging. Atomic absorption analysis reveals the presence of small quantities of metallics which are not responsible for the early stage of deactivation, but slowly lead to a permanent decline in activity.