Since phosphomolybdic acid (PMA) is known to be transformed into an active catalyst for liquefying coal and converting the residual product to distillate material, its conversion under conditions similar to those used in liquefaction has been investigated. The form of active catalyst formed from PMA under conditions of liquefaction is unknown. This paper reports the reaction of PMA observed in He, H-2 and H2S-H-2 at temperatures up to 450 degrees C. PMA is stable in both He and H-2 up to 200 degrees C. However, when H2S is present, sulfur is incorporated rapidly into the structure. The amount of incorporated sulfur increases both with temperature and exposure to H2S. At temperatures up to 300 degrees C, the maximum incorporation of sulfur was approximately 5 wt.% to give a material having a S/Mo atomic ratio of 0.27. Exposure to a large excess of H2S did not further increase incorporation of sulfur. At 450 degrees C, which is the temperature required for liquefaction of coal, approximately 20 wt.% sulfur was incorporated to give a S/Mo atomic ratio of 0.88. In this sample, the remaining oxygen was present as P2O3. IR and Raman spectra showed the presence of MoS2, although insufficient sulfur was present to completely convert Mo to its disulfide. When H2S is present, even at 125-150 degrees C, the crystalline structure collapses destroying the Keggin structure. The rate of sulfur addition to PMA is much faster for MoO3, than for MoO2.