Cloud and spinodal loci have been obtained as functions of pressure (P), temperature (T), polymer segment number (r), segment fraction (psi), and H/D substitution on solute (z(D)) or solvent (y(D)) for polystyrene-h/propionitrile-h (PSh/PNh) solutions, polystyrene-h/propionitrile-2,2d(2) solutions (PSh/PNd), and polystyrene-d/propionitrile-h solutions (PSd/PNh). A light scattering technique was employed. An increase in pressure or an increase in the H/D ratio in the solvent (or decrease in the polymer H/D ratio) increases the region of miscibility. The solutions show upper and lower branches which may join at hypercritical points depending on the pressure, molecular weight, or solvent H/D ratio. A least-squares technique to fit experimental data to the continuous mean-field thermodynamic description of polymer-solvent demixing developed by Luszczyk, Rebelo, and Van Hook (Macromolecules 1995, 28, 745) is described and then illustrated by application to the PS/PN data.