This paper presents the first analysis of the mathematical structure of a system of conservation laws modeling compositional flow of four components in three phases. The phase behavior that results from assuming the equilibrium volume ratios of the components in the phases are fixed (constant K-values) when up to three phases may form, is studied. We parameterize the equations in the three-phase region and show that within the three-phase region two of the characteristic curves can be found using three-phase immiscible flow theory. The third eigenvalue can also be found analytically when the K-values are constant. We show that the eigenvalue problem given by the conservation law has a discontinuity at the boundary of the two- and three-phase regions. Finally, the loss of strict hyperbolicity is discussed.