A process for the simultaneous desulfurization and hydroconversion of heavy carbonaceous feeds, including various sulfur-containing heavy petroleum oils, is disclosed. These feedstocks are contacted with potassium sulfide in a conversion zone maintained at elevated temperatures and in the presence of added hydrogen. In this manner, the feeds are substantially desulfurized, and significant upgrading of these feeds is also obtained as demonstrated by decreased Conradson carbon, increased API gravity, and the conversion of substantial portion of the 1,050.degree. F.+ portion of these feeds. In a preferred embodiment, such a process is disclosed employing a combination of potassium sulfide and sodium sulfide, and in particular these processes include procedures for the regeneration of the sulfides and their recycle to the conversion zone.