The phase behavior of a system of five or more components cannot be totally represented in three dimensional space. The most common method of projection representing the phase behavior of such systems employs a smaller number of rsquo;rsquo;pseudocomponents.rsquo;rsquo; Such a representation must be chosen with care because the multiphase region is less well defined than that of the corresponding true component system. Several examples are discussed in which ambiguous results are obtained when a system is treated as if it consisted of an incomplete set of pseudocomponents. The ordinary lever rule has been generalized to apply to threehyphen;phase systems, making it possible to calculate the mass fractions of three phases from their compositions. For true ternary systems this lever rule can be inverted to yield the compositions of the three phases in terms of the mass fractions at three observation points. For more than three observations on pseudoternary systems the deviations from true component behavior can be estimated from the spread in the calculated phase compositions obtained from different triplets. The method is illustrated for a true ternary system and two pseudoternary systems. The lever rule and its inverse may also facilitate an approach to a tricritical point where three fluid phases become identical.
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