A copper(I) chloride-N,N,N＇,N＇-tetramethyl-1,2-ethanediamine complex supported on silica gel was prepared by mixing porous silica gel with an acetonitrile solution containing the complex composed of copper(I) chloride (CuCl) and N,N,N＇,N＇-tetramethyl-1,2-ethanediamine (M4en) under nitrogen, followed by removal of liquid phase under reduced pressure. The resulting silica gel, 6.0 g, supported 4.92 mmol of 1:1 CuCl-M4en complex. It rapidly adsorbed carbon monoxide (CO) at 30 degrees C under 1.0 atm of CO, and the equilibrium amount of adsorbed CO was 4.30 mmol (87 mol% against supported complex). The adsorbed CO was rapidly desorbed by maintaining it at 120 degrees C under 1.0 atm or at 30 degrees C under 0.1 mmHg. The adsorption-desorption cycle was repeated without any deterioration. The equilibrium amount of adsorbed CO (4.30 mmol) was 11.8 times that of adsorbed carbon dioxide (0.37 mmol), and 40.9 times that of adsorbed methane (0.11 mmol) at 30 degrees C under 1.0 atm. The reversible and selective adsorption of CO was attributed to formation of 1:1:1 CuCl-M4en-CO complex. When the adsorbent contacted with oxygen, the CO-adsorbing ability decreased. The adsorbing ability was completely recovered by maintaining the adsorbent at 120 degrees C under 1.0 atm of CO for 3 h.