The effect of pressure on the tricritical point ͑TCP͒ of a ternary system, polystyrene ͑PS͒ IϩPS IIϩmethylcyclohexane, was studied by numerical analysis of the generalized FloryHuggins equation for the Gibbs free energy. For the ternary system, the molecular weight of PS II M 2t , total volume fraction of PS st , volume fraction of PS II with respect to total volume of PS 2t , and temperature T t at TCP were calculated for various values of the molecular weight of PS I M 1 in the pressure range from 0 to 100 MPa. As M 1 increases, the ratio r t ϭM 2t /M 1 and st decrease, while T t and 2t increase monotonically irrespective of p. The curves of r t and st vs p have a maximum, while the curves of T t and 2t vs p have a minimum, irrespective of M 1 . For small M 1 these curves are nearly flat and roughly symmetrical with respect to the extrema. From these calculated results it is predicted that the tricritical solution at atmospheric pressure remains very near the tricritical state along the cloud-point curve in the temperature vs pressure diagram.