The adsorption of CO on the faujasite-type zeolite Sr−Y generates four types of carbonyls:  the Sr(CO)2+ species, which has a characteristic IR absorption band at 2191 cm-1; the Sr(OC)2+ species at 2095 cm-1; Sr(CO)22+, which has a single mode at 2187 cm-1; and Sr(CO, OC)2+, which has a mode at 2098 cm-1. The use of a variable-temperature IR cell allowing the simultaneous measurements of temperature, pressure, and coverages (intensities) of CO species to be made has allowed the determination of the standard enthalpy and entropy of formation of each species, so obtaining a fairly thorough thermodynamic characterization of the CO/Sr−Y system. C- and O-bonded carbonyl species were found to be in a temperature-dependent isomerization equilibrium, which was also studied by means of variable-temperature IR spectroscopy.