As a model system for new polymer-separation processes utilizing compressed gases the system polystyrene-cyclohexane-carbon dioxide with monodisperse polymers (M-w/M-n=1.09) was investigated experimentally at temperatures up to 250 degrees C and pressures up to 160 bar. Small amounts of carbon dioxide shift the lower critical solution temperature (LCST) to dramatically lower temperatures. When looking at the pressure-concentration representation there is a striking similarity between the effect of temperature and the effect of gas content on the shape of the two-phase region. Pressure has a pronounced effect on the chain partitioning of polystyrenes of different molecular weights in the two liquid phases as determined by gel permeation chromatography. The results are discussed in the context of industrial application.