The phase equilibrium conditions and entropy balance equations for multicomponent fluid mixtures are expressed with a density-based formalism (isochoric thermodynamics), and isentropes in the one- and two-phase region are computed from equations of state; here the Peng-Robinson equation is used as an example. Griffiths theorem-one- and two-phase isentropes meet at a maxcondenbar point (pressure maximum of an isopleth) with equal slopes-could be confirmed. For chemically similar compounds at subcritical conditions, the resulting isentrope patterns are similar to those of pure fluids. If one of the components is supercritical, it is possible that, along a part of a two-phase isentrope, the liquid phase has a higher molar entropy than the vapor phase (entropic inversion). The phenomenon not only poses a numerical problem, but is also relevant for the question whether a two-phase isentrope can run into the llg three-phase curve.