In the production of gas mixtures containing CO and H2 in the volume ratio of about 1 : 2 for synthesis purposes by the gasification of fuels under several atm. pressure by means of O2-steam mixtures, the proportion of O2 used being about 0.1-0.2 m.3 per m.3 of gas produced (calculated at normal temperature and pressure), CO2 is added to the gasification medium in amounts of 0.5-3 times the vol. of O2 employed. In an example, coke is gasified under 10 atm. pressure with a gas mixture comprising about 0.13 m.3 O2, 0.65 kg. steam and 0.2 m.3 CO2 to produce a gas mixture in which the CO and H2 are 21 and 42%, respectively. CO2 is washed from the gas mixture under pressure, and a part is returned to the producer for addition to the gasification medium. Inferior tars or oils or gaseous hydrocarbons may be added to the gasification medium and are cracked in the product.