Abstract: Coal, petroleum residuum and similar carbonaceous feed materials are subjected to hydroconversion in the presence of molecular hydrogen to produce a hydroconversion effluent which is then subjected to one or more separation steps to remove lower molecular weight liquids and produce a heavy bottoms stream containing high molecular weight liquids and unconverted carbonaceous material. The viscosity of the bottoms streams produced in the separation step or steps is prevented from increasing by treating the feed to the separation step or steps with hydrogen sulfide gas prior to or during the separation step or steps. The viscosity of the heavy bottoms stream produced in the final separation step is also controlled by treating these bottoms with hydrogen sulfide gas.