Process for changing composition of gaseous feed mixture containing hydrogen and carbon monoxide in a first molar ratio of hydrogen to carbon monoxide, comprises: a) partially condensing at least a portion of gaseous feed mixture b) separating the partially condensed feed mixture by non-fractionating phase separation (C101, C102, C103, C104, C105) to provide a gaseous product stream, containing hydrogen and carbon monoxide at different hydrogen-carbon monoxide molar ratio, and a carbon monoxide-rich process stream c) warming the gaseous product stream without further cryogenic separation. Also claimed is an appts. for changing the composition of a gaseous feed mixture.