A gas mixture containing oxides of carbon and hydrogen, and preferably substantially free from hydrogen sulphide is passed at a temperature within the range 350-600 DEG , preferably 400-500 DEG C., successively through three zones containing fluidized materials, the first a spent nickel-magnesia-kieselguhr methanization catalyst, the second an alkaline earth metal compound, and the third a nickel-magnesia-kieselguhr methanization catalyst, oxygen being added to the exit gas from the first zone in amount sufficient to oxidize the hydrogen sulphide therein. Pressure may be atmospheric but is preferably elevated, e.g. 1-35 kg/cm.2. The initial mixture, freed from H2S and having a CO : H2 ratio from 1 : 10 to 1 : 0.5, has its organic sulphur converted to H2S in the first zone while unsaturateds are hydrogenated. The second zone may contain calcium oxide or carbonate, suitably with oxygen-containing compounds of Fe, Al, Ti or Si as activators. Oxygen or air introduced equivalent to, e.g., 3-30, preferably 12-20, mols. O2 per mol. H2S. Two or more reactors in series are preferably used. Methanization is then effected in the third zone. Steam is added if the gases are deficient in hydrogen, e.g. 0.2-3 mols. steam per mol. excess CO, and the CO2 formed may be removed to obtain a product containing above 90 per cent methane. Gas recycling may be used in the third zone. The three zones may be superposed in a common casing. Gas loads may be 300-5000 or more vols/vol. catalyst/hr. Gas velocities of 2-15 cm/sec. fluidize the methanization catalysts while the alkaline earth compounds require 8-60 cm/sec. By periodically increasing the gas velocity, methanization catalyst may be carried over to permit replacement by fresh catalyst. In an example, water gas is treated, temperatures being 450 DEG C. in the first two zones and 380-420 in the last, pressure being 11 kg/cm.2.