AbstractAcoustic absorption and adiabatic compressibility measurements are reported on solutions of polystyrene (Mn = 89,000) in toluene and cyclohexane. The data in toluene cover a temperature range from 293 to 343°K and a concentration range of 10–400 Kg m−3 (1–40 wt%). The dependence of acoustic absorption on concentration was found to be linear up to 100 kg m−3, which corresponds to the concentration at which polymer–polymer interactions cause significant changes in the specific viscosity‐concentration relationship. Up to 200 kg m−3 the data could be fitted to computations based on an artificial separation of the dispersion into contributions from viscoelastic and segmental processes, using parameters obtained from a study of narrow molecular weight distribution samples at 25 kg m−3. However, neither approach was capable of describing dispersions in the 300, 400 kg m−3 solutions. The modification of the relaxation spectrum observed at the highest concentrations is ascribed to volume and entropy changes associated with alterations of the local environment around a segment of the polymer chain. These changes have their origin in interchain penetration and polymer–polymer contacts, and indicate that ‘entanglement’ is primarily entropic in effect.The adiabatic compressibility exhibited similar deviations from a simple concentration dependence, and allowed estimation of an incompressible volume increment associated with polymer–polymer interactions in the high‐concentration entangled matrix. However, the adiabatic compressibilities of solutions of polystyrene, 10–15 kg m−3, in cyclohexane showed no deviations from simple behavior in the region of the theta temperature. Measurements of the adiabatic compressibility of polystyrene in mixtures of cyclohexane‐toluene have been used to obtain the relative magnitude of solvent and polymer contributions to the excess compressibility.