A new apparatus is described for easy and quick determination of partition coefficients in a system containing an aqueous and a supercritical phase, and an organic substance, The partition coefficient in this case is defined as the ratio of molar fractions of a substance in two different phases in equilibrium. The apparatus consists of a high-pressure cell of 200-mL volume. Equilibrium is attained by recirculation of the fluid phases. Samples can be taken from either phase by using six-way sampling valves, Quantitative analysis is carried out either by UV-spectroscopy or by gas chromatography. In the measurements, carbon dioxide was used as the supercritical fluid. in studies of partitioning behavior of the organic compounds, phenol, benzoic acid, benzyl alcohol, 2-hexanone, vanillin, and caffeine. The experiments were carried out at temperatures ranging from 313 to 333 K and pressure of 8 to 30 MPa. Partition coefficients between 0.2 and 1.5 were found for phenol which roughly match the data previously reported by other authors. Partition coefficients of benzyl alcohol and benzoic acid were found to be in a similar range, whereas tl-lose of 2-hexanone turned out to be between 10 and 140. The partition coefficients obtained ranged from 0.02 to 0.25 for caffeine and 0.2 to 3 for vanillin.