The effect of end group substitution on the phase behavior of blends of poly(styrene) (PS) and poly(vinyl methyl ether) (PVME) is characterized by determination of experimental cloud point curves. Incorporation of a fluorosilane end group on PS increases the lower critical solution temperature by 10 degreesC indicating enhanced miscibility. The end group effect is modeled by a modification of Sanchez-Lacombe-Balazs (SLB) theory that employs binary interaction theory to account for the end group effect. The increased miscibility is quantitatively predicted by the modified theory when binary interaction parameters are calculated by group contribution methods. SLB theory is also compared to phase diagrams of PS/PVME blends previously reported by Halary et al., and Yang et al., but fails to reproduce the observed behavior over the entire molecular weight range. Optimal fits of these data yield interaction energy parameters that are molecular weight dependent, suggesting that the SLB theory may require, molecular weight dependent equation of state parameters in order to reproduce the phase behavior of polymer blends.