The intrinsic viscosities of five polystyrene (PS) fractions in mixed solvents of methylethyl ketone (MEK) and hexane were determined.When the intrinsic viscosities are plotted against the solvent compositions,there appears a maximum at the volume fraction of hexane γ=0.13. The light scattaring measurements of a PS fraction in seven mixed solvents of varied compositions show that the values of the second virial coefficient A,also pass through a maximum at γ=0.13. The above results are inter-preted by means of solubility parameters,that of PS lies between those of MEK and hexane.From our experimental results.a solubility parameter of 8.8 for PS is suggested.The θ-composition of PS-MEK-hexane at 30.0℃ was determined both from light scattering and viscosicy measurements;the results obtained are in agreement with each other and γθ=0.45.