A process for the simultaneous partial oxidation and desulfurization of sulfur and silicate-containing solid carbonaceous fuel for the production of gaseous mixtures comprising H.sub.2 and CO and containing less than about 0.05 volume % of H.sub.2 S and COS. In the process, an aqueous slurry of the solid carbonaceous fuel and a copper-containing compound are reacted by partial oxidation in the reaction zone of a free-flow unobstructed down-flowing vertical refractory lined gas generator with a controlled amount of free-oxygen containing gas and with optionally a temperature moderator so that an equilibrium oxygen concentration is provided in the gas phase in the reaction zone having a partial pressure which is less than about 10.sup.-6 atmosphere. The O/C atomic ratio is in the range of about 0.5 to 2.0. The H.sub.2 O/solid carbonaceous fuel weight ratio is in the range of about 0.2 to 0.7. Further, the total moles of copper in the reaction zone is at least equal to about 1.0 times the moles of sulfur in the solid carbonaceous fuel. The partial oxidation and desulfurization reactions take place simultaneously at a temperature which produces fly-ash or molten slag at an increased thermal efficiency. At least about 90 wt. % of the sulfur in the solid carbonaceous fuel in the reaction zone is converted into copper oxysulfide particulate matter which leaves the reaction zone along with the fly-ash or molten slag entrained in the hot raw effluent gas stream comprising H.sub.2, CO, CO.sub.2 at least one gas selected from the group consisting of H.sub.2 O, N.sub.2, CH.sub.4 and A; and less than about 0.05 volumne % of H.sub.2 S and COS. The mole ratio in the reaction zone for H.sub.2 O/H.sub.2 is in the range of about 0.4 to 3.0.