In support of its objective to conserve mineral resources by minimizing premature failure of materials, the Bureau of Mines conducted a cooperative study with the U.S. Department of Energy on the erosion and abrasion resistance of hard materials for valves in coal conversion systems. This report describes a newly developed erosion-testing apparatus and presents data on the erosion resistance of over 200 materials. Erosion resistance of most metals was comparatively low. In contrast, ceramics and cermets such as b4c, wc, sic, si3n4, and tib2, when fabricated to minimize porosity, displayed greater than five times the erosion resistance of metals. Coatings such as boron diffused into mo and wc, chemical-vapor- deposited ticn, and electrodeposited tib2 were highly erosion resistant if applied in thicknesses ranging from 60 to 75 um. Erosion resistance of cemented carbides was inversely related to metal binder content. This report was prepared in cooperation with the U.S. Department of Energy.