Carbon monoxide adsorbed at a low-temperature on the zeolite Li-ZSM-5 forms Li+ . . . CO and Li+ . . . (CO)(2) species characterized by C-O stretching bands at 2195 and 2187 cm(-1), respectively. These C-bonded carbonyls are in a temperature-dependent equilibrium with Li+ . . . OC and CO (. . .) Li+ . . . CO species having O-bonded CO. By means of variable-temperature FTIR spectroscopy, the enthalpy change involved in the corresponding isomerization process was found to be DeltaHdegrees = 7.8 U mol(-1) for the monocarbonyl and DeltaHdegrees = 5.1 U mol-1 for the dicarbonyl. Therefore, although C-bonded species were always found to show a higher cation-CO interaction energy than O-bonded species, the difference is smaller in the case of dicarbonyls. (C) 2002 Elsevier Science B.V. All rights reserved.