Phase equilibria in polymer blends are studied using the polymer reference interaction model (PRISM) theory and expanded Gibbs ensemble (ECE) simulations. The fluctuation theory of Kirkwood and Buff is generalized to polyatomic fluids and used in conjunction with self-consistent PRISM; the formalism discussed here permits direct calculation of the volume and Gibbs free energy of mixing of polymer blends at constant temperature and pressure. Theoretical predictions for the volume of mixing are in good agreement with constant pressure simulations of binary polymer blends. Miscibility diagrams for polymer blends predicted by die theory are consistent with the results of EGE simulations.