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, the solid carbonaceous fuel and a calcium-containing compound are reacted by partial oxidation in the reaction zone of a free-flow unobstructed gas generator with a controlled amount of free-oxygen containing gas and 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.-12 atmospheres. The total moles of calcium 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 is about 10.degree. to 200.degree. F. below the softening temperature of the ash in the solid carbonaceous fuel and below about 2000.degree. F. at an increased gasification efficiency. The sulfur in the solid carbonaceous fuel in the reaction zone is converted into calcium sulfide particulate matter which leaves the reaction zone along with the fly-ash entrained in the hot raw effluent gas stream.