Abstract: Process for the production of hydrogen by reacting combustible substances with steam and converting carbon monoxide present in the resulting gaseous mixture into hydrogen and CO.sub.2 by reaction with steam. The amount of steam used in the conversion is increased by contacting said gaseous mixture before the conversion with a stream of water preheated by the gas obtained from the conversion reaction or by other sources of heat at low temperature, to achieve a saturation quench of said gaseous mixture by the conversion of the heated water with sensitive heat of the gaseous mixture into steam, and transfer of the heat from after to before the conversion. The saturated gaseous mixture is then heated to the temperature necessary for the conversion reaction.