The phase equilibrium of the system consisting of polydisperse polystyrene in liquid 1,1,1 -chlorodifluoroethane (R142b) was investigated in the temperature range 395-454 K and pressure range 6.8-30 MPa. Partition coefficients were determined from the overall polystyrene concentration in the solvent-rich phase measured by u.v. absorption spectroscopy and from the polymer distributions in the two coexisting phases of the two-phase system at equilibrium. The partition coefficient of a polymeric pseudocomponent (corresponding to a slice of the polymer chromatogram) is defined as the ratio of its mass concentration in the solvent-rich phase to its weight fraction in the polymer-rich phase on a solvent-free basis. At constant temperature and pressure, the partition coefficient of a pseudocomponent is shown experimentally to be independent of the polymer distribution in the polymer-rich phase. Therefore the mass-based partition coefficient, as defined, of a pseudocomponent can be identified with the mass concentration of this pseudocomponent in the solvent-rich phase for an equilibrium in which only a pseudocomponent and the solvent are involved. For given experimental conditions, the partition coefficients are found to have a log-linear dependence on the chain molar mass. From the temperature and pressure effects on the partition coefficients, the potential of R142b for fractionation is discussed. (C) 1997 Elsevier Science Ltd.