A process for separating carbon monoxide from a gas mixture is presented. In this process carbon monoxide contained in the gas mixture containing carbon monoxide and up to approximately 40,000 ppm of water can be effectively separated by contacting the gas mixture with an absorbing solution containing (a) at least one copper(I) halide, (b) at least one aluminum(III) halide and (c) at least one compound having at least two benzene nuclei in one molecule thereof. The component (c) is selected from the group consisting of (A) the compounds having the general formula (I): ##STR1## wherein R.sub.1 is hydrogen or an alkyl group, R.sub.2 and R.sub.3 are independently hydrogen, an alkyl group, or an aryl group, i is 0, 1, 2, 3, or 4, and m and n are independently 0, 1, 2, or 3, (B) the oligomers styrene or the derivatives thereof, (C) the polymers of styrene or the derivatives thereof and (D) copolymers of (i) styrene or the derivatives thereof and (ii) ethylene or the derivatives thereof and, optionally, an aromatic solvent, without causing the decrease in the carbon monoxide absorbing and releasing capacities of the absorbing solution.