Gas chromatography was used to measure activity coefficients, ϒ ∞, partial molar free energies of mixing, Δ G − ∞ 1, Flory-Huggins interaction parameters, χ ∞, and its enthalpic and entropic contributions, of a number of solutes on five single and fourteen mixed stationary phases at 120°C. Solubility parameters of the stationary phases, δ 2, were deduced by the method of DiPaola-Baranyi and Guillet for the different solute-stationary phase combinations. The polymer-polymer interaction parameter, χ′ 2,3, was also calculated for the various mixed systems.