CO adsorbents were prepared by forming small CuCl crystallites (similar to 160 Angstrom) on the surface of an intermediate density silica gel. CO adsorption by the adsorbents is carried out by converting the CuCl crystallites into a bulk [Cu(CO)Cl] complex under moderate CO pressure (e.g., 2.7 atm at 30 degrees C). The bulk [Cu(CO)Cl] decomposes when the gas phase CO pressure is below the equilibrium CO pressure (e.g., 0.5 atm at 30 degrees C). This unique CO adsorption mechanism results in a CO isotherm consisting of separate adsorption and desorption branches, i.e., a large hysteresis feature. The threshold pressure of [Cu(CO)Cl] formation appears to be a function of the size of the CuCl crystallites.