In a two-phase (supercritical fluid-fluid) system, the rate of conversion of a bimolecular reaction A + B --> AB reacting in the layer adjacent to the interface and in the bulk phase was determined. The mass transfer associated with the chemical reaction was measured in a stirred and thermostatically controlled cell varying reaction temperature, pressure and stirring speed. As model system, the Diels-Alder reaction between 9-hydroxymethylanthracene and maleic acid was chosen in the solvent pair supercritical propane/water. The cell was developed for high pressures and temperatures. The reaction was monitored in both phases with on line UV-spectroscopy. The experimental results are compared to simulated data obtained by using a film model. An improved theoretical model describing the changes in concentration of the reactants in the two phases is presented.