The invention relates to a method for simultaneously producing hydrogen and carbon monoxide consisting in generating a synthesis gas and in processing it by decarbonising and removing water and remaining carbon dioxide by passing said gas through a bed of adsorbents, in separating remaining components by forming at least one H.sub.2 rich flow, a CO flow containing at least one type of impurity selected from nitrogen and argon, a methane-rich purge gas flow and a flash gas flow. The inventive method also consists in regenerating the bed of adsorbents by passing a regeneration gas comprising at least one non-zero proportion of the formed H.sub.2 flow and in recycling at least the purge and flash gases for feeding the synthesis gas generation stage.