Phase diagram of PS-cyclohexane system was examined using cloud point measurements, light scattering,calorimetry,and the ratio between the volumes of coexisting phases. It was found that the boundary curve determined calorimetrically at a scanning rate 10(-6) K/s coincides with the spinodal in the range of medium solution concentrations; for dilute solutions, the curve shows a horizontal portion at 303.86 K. In the range of medium solution concentrations, the right-hand branch of spinodal shows an inflection. The specific features of phase diagram ara due to the coil-globule conformational transition. The transition, which is observed at phi(2) <5.5 x 10(-3), takes place both in the stable (above the binodal) and metastable (between the binodal and spinodal) regions and is, in fact, a first-order phase transition. The temperature at which the chains collapse is independent of the concentration of solution.