Abstract Mixtures of water and a hydrophilic organic solvent, e.g. an alcohol, a ketone, or a carboxylic acid, reveal a complex phase behaviour when pressurized with near critical carbon dioxide at temperatures near the critical temperature of carbon dioxide. Although water and the hydrocarbonaceous solvent are completely miscible, by pressurization with carbon dioxide a liquid–liquid phase split is observed, resulting in a ternary liquid–liquid–vapor equilibrium. In some cases even a four-phase equilibrium is observed. Such equilibria are discussed for systems with the organic solvents methanol, ethanol, 1-propanol, 2-propanol, acetone and propionic acid. That phase behaviour is modelled using the Peng–Robinson equation of state with several mixing rules. A short description of the mathematical tools for such phase equilibrium calculations is given. From pure component and binary data alone, the calculations usually result in a qualitative agreement with the experimental data, i.e. typical effects like for example the existence of ternary critical endpoint lines and tricritical points can be predicted. However, in most cases the quantitative agreement with experimental results is poor. It can be considerably improved by fitting some interaction parameters to ternary high-pressure three-phase equilibrium data.

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