The statistical associating fluid theory (SAFT) equation of state has been shown to accurately predict the effects of molecular weight, intermolecular association, and compressibility on the phase behavior of mixtures containing solvents, monomers, and polymers. In this paper, we briefly review the physical basis for the SAFT approach and present recent extensions of the equation of state. An advantage of theoretically based equations of state is that the theory can be systematically improved to address weaknesses or to extend the theory to more complex fluid systems. Recent developments beyond SAFT allow us to predict the effects of multiple polar groups on the phase behavior of polar copolymers. model the effects of intramolecular association, and efficiently calculate phase equilibrium for polydisperse polymer solutions. These points are illustrated through comparisons with phase equilibria data for monomers, solvents, and copolymer systems. (C) 2003 Elsevier B.V. All rights reserved.