Reinforcing phases used to strengthen a metal matrix cannot be appreciably chemically reactive for the times and temperatures expected during the service life of the composite. Where accurate phase diagrams exist that describe the interactions of the metal matrix and the reinforcing phase, the evaluation process can be completed with these diagrams and, if reactivity is encountered, with appropriate kinetic parameters. These complete phase diagrams are rarely available for complex materials systems. Therefore, we have adopted several ways of using available thermochemical data to estimate the compatibility of various metal matrix composite systems. Usually evaluations involve comparing matrix component activities, using reinforcing phase component thermochemistry to decide whether or not a reaction is expected by examining product chemical activities. Other methods use integral thermochemical properties of reactants and possible products in estimations. In either case a single thermodynamically favored reaction will disqualify a reinforcing phase on an energetic basis if not a kinetic basis. Where no disqualifying reactions are found, the reinforcing phase-metal matrix system is considered a candidate to be proved or disproved in experimental studies which may reveal reaction products not included in the extensive but still limited thermochemical literature. This approach has been advantageously applied to several systems. The study of Ti-Al metal matrices with selected reinforcing phases is discussed as an example.