The objective of this proposed effort is to demonstrate the promise of advanced C/SiC and SiC/SiC composites having improved environmental durability and longer life at higher allowable stress levels without using problematic external barrier coatings. Both oxidation inhibited C/SiC and SiC/SiC composite material systems are proposed for this effort on the basis that: (1) C/SiC offers the highest use temperature and lowest cost of all currently available refractory composite systems, and (2) SiC/SiC offers the highest durability and longest life. Each material system offers unique performance/cost benefits and limitations, and each has been identified as a viable candidate for advanced propulsion and thermal protection system component applications. Oxidation resistant C/SiC and SiC/SiC composite plates will be fabricated incorporating a recently developed, 2nd generation oxidation inhibited matrix produced by chemical vapor infiltration (CVI). Test samples from each material system will be prepared and experimentally evaluated in high-temperature tensile stress oxidation environments. The tensile stress rupture results will be compared to "baseline" uninhibited C/SiC and SiC/SiC composites to establish the performance benefits of the proposed approach.