Conversion of phosphomolybdates under typical hydroprocessing conditions were investigated since they are known to be transformed into active catalysts for hydroconversion processes. This paper reports the reaction of phosphomolybdic acid (PMA), nickel phosphomolybdate (NiPM), cobalt phosphomolybdate (CoPM) and potassium phosphomolybdate (KPM) in He, H-2 and H2S-H-2 at temperatures up to 450 degrees C. The phosphomolybdates sulfided at 300, 350, 400 and 450 degrees C were analyzed by FT-IR, Raman, XRD and XPS. These phosphomolybdates are stable in both He and H-2 until they lose their crystalline water. However, in the presence of H2S, sulfur is incorporated into the structure rapidly. Although sulfur incorporation up to 300 degrees C is low, it increases considerably as the sulfidation temperature increases. The S/Mo ratios in the sulfided materials show that sulfur uptakes into the structures are less than two for PMA, NiPM and CoPM whereas it was 2.1 for KPM at a sulfidation temperature of 450 degrees C. Spectroscopic analyses suggest that the products from the sulfidation of phosphomolybdates may be NiO or CoO, MoS2, P2O3 or K4P2O7 and a molybdenum oxide (MoOx) or a molybdenum oxysulfide (MoOxSy), The oxidation state of Mo decreased to 3.8+, 4.8+, 4.6+ and 5.4+ at 300 degrees C, and 3.8+, 4.2+, 4.2+ and 4.4+ at 450 degrees C for the sulfided PMA, NiPM, CoPM and KPM materials, respectively. Results indicate that the active catalyst in hydroconversion processes does not resemble the Keggin anion. The active catalysts may be a mixed oxisulfides and some bulk metal sulfides.