Light-scattering and phase-separation experiments were performed on cyclohexane solutions of four narrow-distribution samples of four-arm star polystyrene with molecular weights of 8.5 x 10(4) to 1.4 x 10(6), and the results were compared with literature data for linear polystyrene in cyclohexane. The upper critical solution temperatures T-c for the star polymer were systematically lower than those for the linear polymer, showing a considerable effect of chain branching on phase separation. The apparent second virial coefficients J of light scattering for the two polymers, plotted against phi/P-0.1 at a fixed temperature below Theta, happened to form a composite curve at high polymer concentrations, regardless of the relative degree of polymerization P (phi denotes the polymer Volume fraction), whereas they appreciably differed at low concentrations, reflecting the finding that the (true) second virial coefficient for the star polymer is significantly larger than that for the lineal polymer below Theta. The chemical potentials of the solvent and solute components derived from the J data were shown to explain the phase diagrams of the two systems fairly well. Thus it was concluded that the difference in T-c between four-arm star and linear polystyrenes arises primarily from the difference in J in dilute solution.