A modified Flory-Huggins model is presented, considering a concentration- and temperature-dependent interaction parameter, and using the methodology of Continuous Thermodynamics to take into account both polydispersity and its effect on phase equilibrium of polymeric systems. This model describes all commonly found, as well as other unusual polymer + solvent and polymer + polymer, liquid-liquid phase diagrams and is easily extended to take all possible pressure effects into consideration. Modeling and least-squares fit of polystyrene + nitroethane liquid-liquid cloud-point data have produced results in good accord with the experimental ones by using meaningfully physical parameters. These results have been used to discuss polystyrene molecular weight, pressure, and isotopic substitution effects on polystyrene + nitroethane systems. A first-order interpretation of phase equilibrium isotopic substitution effect has also been applied. It combines the simplest form of the Flory-Huggins model with the statistical theory of condensed phase isotope effects.