The phase behaviour of reaction and product mixtures of the partial oxidation of propane in dense carbon dioxide was studied. Critical data of pseudo-ternary reaction mixtures (CO.-propane-air, containing fixed amounts of ca. 5 and 23 mol% of air, respectively) as well as those of multi-component product systems (additionally containing ca. 0.1 mol% of acetic acid/methanol and acetic acid/acrylic acid) were experimentally determined by opalescence measurements and compared to ASPEN PLUS(TM) Computer simulations. The influence of the different components on the mixture behaviour was studied and the accuracy of various equations of state (EOS) was discussed. A good agreement between measured and calculated results was obtained up to air concentration of ca. 5. mol% in the reaction mixture with the Peng-Robinson EOS. Increased air proportions (up to 23 mol%) led to larger deviations. The small proportions of oxygenates appearing in the product mixtures of the reaction studied did not significantly influence critical data. (C) 2002 Elsevier Science B.V. All rights reserved.