A method is presented for the calculation of binary interaction parameters from reacting mixture data. The process studied is the supercritical methanol transesterification of triolein with CO2 as cosolvent. The reaction proceeds through 3 reversible reactions in series, on a solid catalyst (Nafion (R) SAC-13), for which the kinetics is known. To describe the reactor performance, the kinetic constants and the binary interaction parameters for the Peng-Robinson equation of state, are necessary, and were included in the reactor model. Previously published view-cell observations and thermodynamic calculations indicate that no condensation occurs in the reactor. Kinetic and thermodynamic parameters were determined by a global optimisation procedure. From the rate constants obtained, the process exhibits some reversibility, despite the excess of methanol used. Our methodology allows to obtain the binary interaction coefficients for the intermediate reaction species (dioleins and monooleins) that were unavailable so far. The method is sensitive to the errors in the measured concentrations.