A flexural vibration test and associated equipment have been developed to measure accurately the low strain dynamic modulus and damping of composite materials from −200 to over 500°C. The basic test method involves the forced vibration of composite bars at their resonant free-free flexural modes in a high vacuum cryostat-furnace. The complex modulus approach was employed to derive theoretical expressions for composite axial and transverse dynamic properties in terms of the dynamic properties of the constituent phases. The accuracy of these expressions and the flexural test was verified by dynamic moduli and damping capacity measurements on 50 fiber volume percent boron/aluminum composites vibrating near 2000 Hz. Whereas the properties of the highly anelastic fiber were measured on single fibers, the aluminum matrix properties were in most part deduced from the boron/aluminum results. The phase results were summarized to permit predictions of the boron/aluminum dynamic behavior as a function of frequency, temperature, and fiber volume fraction. Analysis of the test data has also indicated several areas in which boron/aluminum dynamic properties might be exploited to evaluate and predict gross composite behavior as a function of environmental conditions encountered in fabrication and use. These areas include fiber-matrix debonding, fiber interaction with matrix and impurities, and thermal treatment effects on the matrix.