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Cyclic and static loading tests were performed on reaction-bonded silicon nitride from 1000–1400 °C in air. This porous, fine-grained material contained no glassy grain-boundary phase and exhibited no slow crack growth at room temperature. Under cyclic loading, the crack-growth behaviour at 1000 °C was similar to room-temperature results; however, at 1200 and 1400 °C crack-growth rates increased significantly. Under static loading, significant crack growth was detected at 1000 °C and increased with temperature. Most of the crack growth under cyclic loading was attributed to slow crack-growth mechanisms, but evidence of cyclic crack-growth mechanisms were also observed. Oxidation played a major role in crack-growth velocity at high temperature.