Cloud and spinodal loci have been obtained as functions of pressure (P), temperature (T), polymer molecular weight (MW), polymer fraction, and H/D substitution on solute (zD) or solvent (yD), for acetone-polystyrene and methylcyclopentane-polystyrene solutions. A light scattering technique was employed. The isotope effects and their pressure dependences are large. An increase in pressure, an increase in H/D ratio in the solvent (or decrease in the polymer H/D ratio), or a decrease in polymer molecular weight increases the region of miscibility. The solutions show both upper and lower branches, which, for acetone, join at a hypercritical point. The cloud point diagrams are highly distorted in the hypercritical region but smooth as one movss away by appropriately varying pressure, H/D ratio, or solute molecular weight. In the vicinity of the multiple critical point, hypercritical distortion for polystyrene/(CH3)2CO solutions causss a highly unusual homogeneous one-phase “hole” in the (T,Wps) cloud point diagram. This “extra” one-phase region appears close to the hypercritical point, and just inside the two-phase region which forms immediately after the system collapses into the “hourglass” configuration. The effect is not seen in (CD3)2CO or mixed (CH3)2CO/(CD3)2CO solutions, but those solutions also show marked distortion in the (T,Wps) plane. H/D substitutions (on either polymer or solvent) cause large differences in cloud point and spinodal temperatures, especially in the vicinity of the hypercritical point. Isotope shifts as large as 40 K were observed on the cloud points of the polystyrene/acetone system. A mean-field formalism to interpret cloud point and spinodal data on liquid-liquid demixing of polymer-solvent solutions exhibiting both upper and lower consolute branches and hypercritical points is developed. The dependence of the phase equilibria on pressure, temperature, segment number, polydispersity, and H/D substitution of either or both solvent and polymer are explicitly considered. Calculations of cloud and shadow curves are reported. The method is based on a Flory-Huggins type continuous thermodynamic representation of free energies in polydisperse polymer solutions due to Ratzsch and co-workers. The computer algorithm PHASEEQr written in QUICKBASICe has been coded for the case of Schulz-Flory segment distribution functions. Calculations interpreting demixing data for polystyrene/acetone and polystyrene/methylcyclopentane solutions over wide ranges of pressure, concentration, molecular weight, and H/D substitution are reported. The isotope effects, which are large, are discussed using Bigeleisen-Van Hook-Wolfsberg condensed phase isotope effect theory. Along the critical line multidimensional classical or nonclassical scaling equations permit economical representation of the data. The scaling approach is developed and compared with the mean-field based interpretation.