A study of the high temperature tensile creep rupture behavior of a commercially available woven [0/90] Nicalon/SiC composite was performed. Tests were performed at temperatures of 500 to 1149°C. At a creep stress of 83 MPa, lives of less than 40 hours for temperatures above 600°C in air were obtained. At the reduced stress level of 69 MPa, lives were twice as long. However, a test conducted at 83 MPa and temperature of 982°C in vacuum yielded a run-out life of more than 1000 hours. Also, the tests performed in air have significantly shorter lives than those reported by the composite manufacturer. Post test investigations focused on identifying the mechanisms responsible for the unexpected difference in life and results are presented. Analytical modeling and supplemental testing revealed an unexpected environmentally assisted material degradation at an intermediate temperature range of 700 to 800°C. Microscopic and chemical analyses indicate that a combination of applied stress and temperature produces an oxidation-embrittlement damage mechanism. The role of the selected specimen geometry and testing conditions, which was instrumental in the detection of this failure mechanism, is thoroughly discussed.