The composition dependence of the thermal-diffusion factor in mixtures of carbon dioxide with helium, neon, and xenon has been studied with the help of an all-glass eight-tube trennschaukel or swing separator. In the case of He–CO2 and Ne–CO2 mixtures, α is found to increase with increasing concentration of the lighter component while in the Xe–CO2 mixture, α plotted against CO2, reaches a maximum value near 40% CO2 and falls off to a minimum value near 70% concentration of CO2. The theoretical composition-dependence curves calculated on the Lennard-Jones (12:6) model are found to reproduce qualitatively the shape of the experimental curves for the He–CO2 and Ne–CO2 systems.