Sulfur-containing petroleum oil feedstocks which include heavy hydrocarbon constituents undergo simultaneous desulfurization and hydroconversion by contacting and reacting such feedstocks with sodamide in the presence of hydrogen and at elevated temperatures. The mixture of reaction products resulting from the above procedure is separated to give a sodium sulfur salt by-product, and a petroleum oil product which has been substantially desulfurized and demetallized, as well as being significantly improved as indicated by a reduced Conradson carbon content and an increased API gravity relative to the feedstock. Sodamide is regenerated from the sodium sulfur salt by-product and can be recycled for reaction with additional feedstock.