The failure of titanium metal-matrix composites (MMCs) strengthened with monofilament carbon-coated sigma SiC fibres has been studied following exposure at a temperature of 900 degrees C. Statistical analysis of the data shows that the time to penetration of the carbon layer on the as-received carbon-coated sigma monofilament (SiC + C) can be used as a failure criterion for these composites. The same technique can also be used as a measure of the effectiveness of different diffusion barrier coatings.