An investigation of the erosivity of particles of several different coals and respective vacuum bottoms from the Exxon Donor Solvent (EDS) coal liquefaction pilot plant of the Exxon Coal Liquefaction Process (ECLP) was carried out. Kerosene and tetrahydrofuran (THF) were used as the liquids in slurries containing 30 weight % particles. The particles were of several sizes, shapes, integrities (fracture strength), and ash contents. It was determined that the primary factors having a direct effect on erosivity were particle size and ash content/composition. Particle shape and resistance to fracturing upon impact had important secondary effects. All of these factors are interrelated and can counter each other under certain conditions. Slurry pot testing proved valuable as a reproducible method for comparative erosion studies, but should not be relied upon to produce quantitative erosion data for equipment design purposes.