DFT calculations were carried out on the carbon and oxygen isotopomers of CO and Ni(CO) 4 and results compared with (Spindel's) experimental results on the carbon and oxygen isotope exchange equilibria between CO and Ni(CO) 4 . The isotopic equilibrium constants, K B , of carbon and oxygen exchange reactions between CO and Ni(CO) 4 complexes were theoretically calculated as the ratio of the reduced partition function ratios (RPFRs) of the 1 3 C/ 1 2 C and 1 8 O/ 1 6 O isotopic pairs for CO and Ni(CO) 4 on the assumption of the internal harmonic vibrations. The agreement between calculated separation factor and Spindel's experimental data are surprisingly good, including the temperature dependence.