Abstract: Carbon monoxide and hydrogen are from a gaseous mixture comprising essentially these two components and methane, by the steps of i) cooling the initial gaseous mixture so as partially to condense it; ii) separating a condensed portion of the mixture from an uncondensed portion containing principally hydrogen; iii) sending the condensed portion to a stripping column (7) to produce a gaseous fraction at the head of the column comprising hydrogen and a liquid fraction at the base of the column containing carbon monoxide and methane; iv) distilling the liquid fraction in a purification column (9) to produce substantially pure carbon monoxide at the head of the column and methane at the base of the column; and v) supplying a portion of the separation energy by a refrigeration cycle using an autonomous refrigeration fluid.