Equilibrium vapor pressures were measured for the systems toluene/poly(dimethylsiloxane) [two different polymers, 35-55 degrees C] and cyclohexane/poly(vinyl methyl ether) [three polymers, 35-65 degrees C] by combining a head-space sampler with a gas chromatograph. The Flory-Huggins interaction parameters chi calculated therefrom as a function of the volume fraction phi(2) of polymer were complemented by literature data on inverse gas chromatography and by chi values at high dilution from osmometry in the former case and from light-scattering measurements in the latter. These results demonstrate that the effects of differing chain lengths can still be felt at phi(2) > 0.5, in contrast to the general perception. This observation is tentatively explained in terms of changes in coil dimension taking place in the region of high polymer concentrations. Furthermore, chi(phi(2)) may exhibit a minimum even with the common polymers of present interest. In all cases the concentration dependence of chi is described well by a series expansion of chi with respect to phi(2) up to the fourth term, if two parameters are adjusted. The temperature dependence of chi at different concentrations demonstrates that the heats of dilution generally increase with increasing phi(2), whereas the noncombinatorial entropy of dilution decreases. This situation leads to a linear interdependence of the enthalpy and entropy part of chi. With the system toluene/poly(dimethylsiloxane) one observes an inversion of the heat effects upon an increase of phi(2) from exothermal to endothermal at ca. 13 vol %.