Most polymeric matrix materials currently in use with boron filaments are based on epoxy resins having limited strength retention characteristics at elevated temperatures. In order to provide materials capable of fulfilling the requirements for a matrix having the optimum properties necessary for short time, elevated temperature composite application in the aerospace field, an intensive development program was undertaken. An effective evaluation of resin systems for composite applications was undertaken by choosing a test configuration which imposed major stresses in the weak direction of the matrix and matrix-fiber interface. This testing concept made it necessary to compare materials having low flexural modulus and tensile values. The evaluation configuration chosen for this study was a 9-ply boron-resin laminate, with the filaments oriented in the ±45 deg direction. Evaluation consisted of determining the flexural modulus of a 1-in.-wide by 4.5-in.-long plate over a temperature range up to 600 F, and observing the reduction in modulus as the temperature increased. One plate could tnus be used to test the entire range of temperatures, and could then be reconfigured for a tension test, permitting maximum utilization of materials. In addition, through other laboratory evaluation techniques and fabrication studies, it was possible to evaluate a considerable number of resin formulations and modifications. In general, it was found that toughness is the key property required of the resin matrix, rather than high modulus and high strength alone. Unmodified phenolic resins were found to be brittle and subject to crazing under thermal stress on cooldown from cure temperatures. Epoxies have relatively low heat distortion temperatures and are not necessarily compatible with heat shield materials cured simultaneously. A phenolic resin modified with epoxy and polyvinyl formal was found to have a combination of properties that approaches an optimum for a moderate elevated temperature (500 F) resin matrix for boron filaments.