A calorimeter based on a design by McGlashan has been used to measure the heat of dilution of non-polar polymer–solvent systems. The influence of concentration and molecular weight of polystyrene on the heat of dilution with toluene has been examined in detail by use of monodisperse polymers with molecular weights from 900 to 98,000. The concentration-dependence of χH, the apparent enthalpy parameter, shows characteristics similar to those which have been reported for more polar systems. The concentration-independent enthalpy parameter χ1 varies from –0·32 to –0·02 with increasing molecular weight. The excess and residual entropy of dilution parameters have been calculated and an attempt made to interpret the results.