Modelling of thermodynamic properties of chlorinated phenols in water and in 1-octane-ol is presented. Activity coefficients at infinite dilution and enthalpies of mixing were considered. The UNIFAC method as presented by Gmehling et al. [J. Gmehling, J. Li, M. Schiller, A modified UNIFAC model : 2. Present parameter matrix and results for different thermodynamic properties, Ind. Eng. Chem. Res. 32 (1993) 178-193] and Larsen et al. [B.L. Larsen, P. Rasmussen, A. Fredenslund, A modified UNIFAC group-contribution model for prediction of phase equilibria and heats of mixing, Ind. Eng. Chem. Res. 26 (1987) 2274-2286] was used. A method combining the group contribution approach with correlation based on physical descriptors yields results within experimental error.