AbstractMeasurements have been made of the osmotic pressure of polystyrene fractions in toluene and methyl ethyl ketone at 25, 50, and 55°. The results can be accurately represented by the equation:\documentclass{article}\pagestyle{empty}\begin{document}$$ \pi = RT\left[ {\left( {{1 \mathord{\left/ {\vphantom {1 {M_2 }}} \right. \kern-\nulldelimiterspace} {M_2 }}} \right) + A_2 c^2 + A_3 c^3 } \right] $$\end{document}The virial coefficients have been evaluated and it is shown in agreement with theory thatA2αM−εwhere ε = 0.15 or 0.33 according to method of derivation ofA2. The results are compared, and shown to be in general agreement with other reported measurements. The heat of dilution of polystyrene in toluene and methyl ethyl ketone was determined from measurements of the temperature coefficient of the osmotic pressure. In the concentration range 3 to 9 × 10−3g./cc. ΔH/ϕ 22where ϕ2is the volume fraction of polymer was approximately zero in toluene and −20 ± 5 cal./mole in methyl ethyl ketone. In part III vapor pressure measurement of polystyrene fractions in acetone, chloroform, and propyl acetate solutions are summarized and the heat of dilution in these solvents derived. The results are discussed in relation to existing theories of the free energy and entropy of dilution of polymers and it is shown that the results cannot be explained in terms of theories based on the lattice mo