A process for the production of demercurized synthesis gas, reducing gas or fuel gas. Mercury-containing fossil fuels e.g. coal were reacted by partial oxidation to produce gaseous mixtures comprising H.sub.2, CO, H.sub.2 O, CO.sub.2, H.sub.2 S, COS, entrained slag and/or ash, mercury vapor, and optionally CH.sub.4, NH.sub.3, N.sub.2 and Ar. Unexpectedly, the mercury vapor was produced in the reaction zone; and it was found to be thermodynamically stable even in the presence of H.sub.2 S under the strong reducing conditions that prevailed in the gas generator. No new sulfides of mercury were formed. The mercury vapors were removed from the main body of the process gas stream in a pressurized solvent scrubber at a relatively low temperature. By this means, the mercury vapor was condensed and simultaneously the mercury and sulfur contents of the clean process gas stream were reduced to low levels. In one embodiment, the last vestiges of mercury were removed from the demercurized product gas stream by carbon sorption.