PURPOSE: To obtain an excellent CO-absorbing solution which scarcely absorbs CO2 and keeps its capacity even if contaminated with water, by adding HCl to a tris (dimethylamino) phosphine oxide solution of cuprous chloride.

CONSTITUTION: The objective absorbing solution is composed of cuprous chloride, tris (dimethylamino) phosphine oxide, and HCl at molar ratios of about 1:(1.3W5.5):(0.02W0.5), especially preferably 1:(1.7W3):(0.05W0.3). When the concentration of HCl is too low, the stability of the solution at high temperature becomes poor, and when it is too high, the equilibrium absorption of CO is lowered. When the concentration of cuprous chloride is too low, the amt. of the equilibrium absorption of CO is reduced, and when it is too high, the liquid becomes too viscous to carry out the practical treatment of the solution. The CO-absorbing solution is suitable for the use in chemical industries, etc. for the separation, concentration and recovery of CO from process exhaust gas.