Gas mixtures are separated by cryogenic separation, preferably a cryogenic condensation separation cycle, into at least a first gas mixture and a second gas mixture having at least one component common with the first gas mixture; at least one gas mixture selected from at least a portion of said feed gas mixture and at least a portion of said second gas mixture is subjected to non-cryogenic separation to provide a separated gas rich in said common component; and said separated gas is added to said first gas mixture to contribute to a product gas mixture. It is particularly preferred to also blend a portion of the second gas mixture with the first gas mixture to facilitate, in conjunction with the separated stream, simultaneous control of both the amount and composition of the first gas mixture product. The process has particular application to the separation of a gas containing primarily hydrogen and carbon monoxide to provide a product gas which consists primarily of hydrogen and carbon monoxide in a molar ratio different from that of the feed gas.