An important challenge in applied thermodynamics is the prediction of mixture phase behaviour without the use of experimental data. Current group contribution methods are sometimes, but not always successful in this regard. In the present work Monte Carlo free energy perturbations are used in calculating the free energies of solvation for pure component and infinite dilution using the OPLS force field. Infinite dilution activity coefficients are calculated by pure simulation and simulations used in combination with experimental vapour-pressures. The activity coefficients are then used to fit the parameters in Wilson's equation thereby giving overall predictions of activity. Results are compared with free energies and activity coefficients based on experimental values. The systems studied are methanol + water, ethanol + water, acetonitrile + water, formic acid + water and ethanolamine (MEA) + water. (C) 2004 Elsevier B.V. All rights reserved.