The equilibrium partitioning of solvent between water and phospholipid vesicles above the phase transition temperature of the vesicles is treated in a manner similar to that previously successful in describing solvent partitioning between water and polymer latex particles. The mixing of solvent and the alkane part of the phospholipid vesicles is described by classical Flory-Huggins theory. The vesicle-surface free energy is described by the Gibbs-Thomson equation. The subsequent model has a single adjustable. parameter, the saturation concentration of solvent in vesicles, that is restricted within physical limits. Comparisons of theory and-experiment show good agreement and allow calculations of vesicle interfacial tensions.