Second and third virial coefficients, A(2) and A(3), as well as the reciprocal of the osmotic compressibility partial derivativePi/partial derivativec up to high concentrations were measured for low molecular weight polystyrenes dissolved in a good solvent toluene, by sedimentation equilibrium. The results were compared with a theory based on the single-contact approximation, where the polymer chain is viewed as a spherocylinder bearing hard-core and square-well potentials, to determine the hard-core diameter and the depth of the square-well potential of polystyrene in toluene. Furthermore, the present results of A(2), A(3), and partial derivativePi/partial derivativec were combined with previous ones for higher molecular weight polystyrenes in toluene to examine application limits of the single-contact approximation. It was found that partial derivativePi/partial derivativec for a polymer concentration higher than ca. 0.1 g/cm(3) is described by the theory of the single-contact approximation, irrespective of the polystyrene molecular weight, indicating that the effect of multiple contacts on partial derivativePi/partial derivativec is screening off in the concentrated region.