Mass spectrometric measurements were carried out on a CO 2 sample prepared by mixing known amounts of ‘natural’ CO 2 with CO 2 enriched in 13C and, to a lesser extent, in 17O and 18O. The sample was in isotopic disequilibrium, i.e. with the isotopic species distribution not obeying the statistical probability. The results indicate that isotope scrambling reactions occur during the measurement which tend to shift the gas isotopic composition towards the equilibrium. The isotope exchange reactions take place via gas adsorption in the mass spectrometer inlet system. A Langmuir-type adsorption model, developed independently to describe SiF 4 adsorption in the mass spectrometer inlet system, is applied to the CO 2 scrambling reactions. The model is capable of fitting the experimental data adequately, and sets of values for the isotope exchange kinetics can be derived from it. Small deviations from the model prediction may be due to isotope fractionations which have not been taken into account and require further investigation.