Experimental information available in the literature on the phase separation behavior of the system trans-decalin/polystyrene as a function of pressure and shear rate ibr different molar masses M of the polymer was used to check to which extent the effects of these two variables can be predicted theoretically. The Sanchez-Lacombe theory served as a basis for the description of the solutions at rest. To account for the nonequilibrium condition, the energy stored by the sheared mixtures while flowing was added to the Gibbs energy of mixing. The phase diagrams measured at rest and under atmospheric pressure can well be described by one parameter (Delta epsilon*, measuring the interaction energy between unlike mers); Delta epsilon* is found to vary linearly with M-0.5. The pressure dependencies of the cloud points (constant M and constant composition) are also modeled appropriately, if Delta epsilon* is allowed to decrease linearly with rising pressure. The predictions concerning the influence of sheer are only qualitatively correct : In accord with the experimental findings, the phase separation conditions are practically identical for the stagnant and for the flowing solutions at low pressures; the homogeneous region is, however, increasingly extended by shear as pressure becomes larger.