Hydrothermal treatment of Ohio Bituminous Coal and Texas Lignite under different conditions of temperature and residence time results in progressive changes in the ash yield and sulfur content of the products. Below 300C the ash yield of both coals is reduced. At higher temperatures, devolatilization of organic matter competes with the removal of inorganic constituents: the ash yield of the severely treated products is higher than that of the starting coal. Similarly, the contents of the different forms of sulfur in the Ohio coal are reduced under moderate hydrothermal treatment, but the effect is counterbalanced under severe conditions, due to the loss of organic matter.