The adsorption of acetone and its co-adsorption with CO were studied on a solid state ion exchanged Cu-ZSM-5 catalyst using in situ Fourier transform infrared (FTIR) spectroscopy. The C-O stretching vibrational band of adsorbed acetone at Cu1+ is centered at 1671 cm(-1), red-shifted by 48 cm(-1) in comparison to that in the gas phase. Adsorbed acetone at the Cu1+ site does not eliminate the adsorption of CO onto the same cationic center (and vice-versa); however, the strength of the CO binding is suggested to be weaker to the {Cu[(CH3)(2)CO]}(+) center than to the adsorbate free Cu1+ site and is manifested by a 28 cm(-1) red-shift in the C-O stretching frequency of adsorbed CO in the former complex in comparison to the latter complex. The position of the C-O stretching vibrational band of adsorbed acetone blue-shifts by about 20 cm(-1) as a result of the presence of CO on the cationic center. The changes in the C-O stretching vibrational frequencies of the absorption features of the two adsorbate molecules can be rationalized by the changes in the electronic environment at the Cu1+ adsorption center.