The tensile properties of three carbon matrix composites reinforced with SiC (Nicalon) fibers (materials A, B, C) have been measured with and without notches. One of the three materials (material B) had a relatively low strength and exhibited notch brittleness. This material had both a high interface sliding stress and a low fiber bundle strength, caused by particulates in the matrix. These characteristics have been shown to result in a change in failure mechanism that leads to the inferior properties exhibited by material B. The notch properties of the higher-toughness materials were shown to involve splitting, which alleviates the notch stress concentration and diminishes the notch sensitivity.