Three samples of anthracite, Middle Mammoth, Lykens Valley, and St. Nicholas have been subjected to temperatures in the range of 400-900 degrees C and pressures of 2,000 to 20,000 psi in hydrothermal pressure vessels. Selected runs were taken to 30 Kb in an opposed anvil device.The altered materials were studied primarily to determine differences in morphology of the grains, their reflectance values and degree of anisotropy. X-ray diffraction analyses were run to determine the mineral assemblages present before and after treatment.The reflectance values of the altered materials increase with temperature while pressure alone appears to have little effect. When temperature and pressure are held constant, reflectance increases with increasing time of the experimental runs. The rate of increase of reflectance is greatest in the temperature range where evolution of volatile matter is also the greatest. As might be expected the highest "rank" coal showed the least change in reflectance under the conditions applied.It was possible to fuse the anthracite particles with temperatures and pressures as low as 400 degrees C and 2,000 psi. Increased retention time at constant temperature and pressure causes an increase in the degree to which the grains are fused. The extreme temperature and pressure conditions to which the anthracites were subjected have caused them to behave in a fashion similar to bituminous coals at much lower pressures and somewhat lower temperatures.Observations made during this investigation correlate with existing theories on coal metamorphosis, pyrolysis and structure, extended from lower rank coals.