A process for the simultaneous partial oxidation and desulfurization of an ash-containing solid carbonaceous fuel comprising (basis solid fuel) 0.2 to 6.0 wt. % sulfur and 0.1 to 30 wt. % of silicate compounds for the production of gaseous mixtures comprising H.sub.2 and CO and entrained molten slag. In the process, the solid carbonaceous fuel and first and second types of supplemental calcium-containing materials 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 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.-9 atmospheres. The partial oxidation and desulfurization reactions take place simultaneously at a temperature above about 1900.degree. F. and about 10.degree. to 100.degree. F. above the fluid temperature of the slag at an increased thermal efficiency. The sulfur in the solid carbonaceous fuel in the reaction zone is converted into calcium and iron sulfide particulate matter which leaves the reaction zone along with the silicates of calcium and iron in the molten ash that is entrained in the hot raw effluent gas stream. Additional desulfurization in a fluidized bed containing calcium-containing material in admixture with carbon-containing material may be used to reduce the amount of H.sub.2 S and COS in the effluent gas stream to less than about 0.05 volume %.