A simple acoustic technique has been used to study the vapour-liquid critical points of the reaction mixtures of hydroformylation in carbon dioxide. The systems CO2 + CO + H-2 + propene and CO2 + CO + H-2 + hex-1-ene have been investigated. These two quaternary systems were measured by the pseudo-binary approach (i.e. the ratio of alkene, CO and H-2 being held constant). The effects of the solutes (alkene, CO and H-2) on the critical points in the CO2-rich fluids are discussed.