The oxidative regeneration of spent cobalt-molybdate and nickel-tungstate catalysts from hydrotreatment of petroleum vacuum gas oil and coal-derived oil was carried out in a fixed bed reactor. Temperature-programmed oxidation studies revealed that oxidation proceeded mainly in two steps (i.e., removal of sulfidic sulfur as SO2 around 500-600 K and removal of carbon as CO2 (CO) around 650-850 K). Carbonaceous materials on the spent NiW catalyst were less aromatic than those on the spent CoMo catalyst, but more severe oxidation conditions were needed for the NiW catalyst because of the lower oxidation activity of NiO and WO3 compared with Co3O4 and MoO3. For the NiW catalyst, EXAFS data revealed that WS2-like structures, which were laterally grown during the hydrotreatment run, were redispersed to nearly the same level as that of the fresh catalysts when carefully controlled oxidizing conditions were used (p(O-2) = 1.5%). XPS data showed that surface compositions of Ni and W were recovered to almost the level of fresh catalysts, but the Ni/W ratio was slightly less than that of the fresh ones. Catalytic activities and selectivities were successfully recovered by low-temperature oxidation. On the contrary, for the CoMo catalyst on which MoS2-like sulfides were laterally grown, some of the Coaggregated to Co9S8, and small amounts of Ni Fe, and V were deposited, it was not possible to recover the same level of structural properties as those of the fresh catalysts. The catalytic activities and selectivities were almost recovered by low-temperature oxidation, while at higher regeneration temperatures there was a slight loss of hydrogenation activity and a large increase in the hydrocracking activity. The thermodynamic analyses were done to better understand the structural changes of the active components and their possible interactions with the deposited metals and the gamma-Al2O3 support during oxidation and subsequent sulfidation.