Previous work has shown that mixtures of multiple phospholipids with cholesterol exhibit phase behavior which may be approximately modeled by a hypothetical binary mixture of cholesterol and an "average"-phospholipid.(1) This study presents a detailed thermodynamic model of such multicomponent mixtures based on regular-solution theory. When the individual phospholipids are treated independently, calculated phase behavior shows systematic deviations from the average-phospholipid model. These deviations may be expressed concisely in terms of the distribution of binary critical parameters of the individual phospholipids, and the deviations are small for mixtures in which the individual phospholipids exhibit similar phase behavior in binary mixtures with cholesterol.