The dense phases of the CO 2 –CCl 4 (I) and CO 2 –CS 2 (II) mixtures have been studied by Raman spectroscopy. Mixture I is found almost ideal and II strongly non-ideal. At high CS 2 concentration in II, the local structure of CS 2 is preserved suggesting a nano-segregation of the liquid phase without demixing whereas in CO 2 concentrated mixtures a diversity of species are present. Thermodynamical considerations together with spectroscopic results show that a subtle interplay between attractive and repulsive interactions leads to this non-ideal behaviour. The connection of these results with the liquid–liquid demixing reported for CO 2 –CS 2 at lower temperature is discussed.