Sulfur-containing petroleum oil feedstocks which include heavy hydrocarbon constituents undergo simultaneous desulfurization and hydroconversion by contacting such feedstocks with alkaline earth metal hydrides or alkaline earth metal oxides, particularly barium hydride or barium oxide, in the presence of hydrogen, and at elevated temperatures. The mixtures of reaction products resulting from the above procedure can be separated to give a petroleum oil product which has been substantially desulfurized and demetallized and significantly upgraded as demonstrated by reduced Conradson carbon content, and increased API gravity and which includes alkaline earth metal sulfide salts from which the alkaline earth metal hydrides or oxides may be regenerated.