The interaction of chemical vapour-deposited (CVD) SiC fibres with H2O at 200 MPa and 400-700 degrees C (673-973 K), was investigated. With increasing temperature and ti me, the smooth surface of the amorphous SiC fibre becomes rough and sponge-like. This modification can be controlled by adjusting temperature and duration of the hydrothermal treatment. CVD SiC dissolves in supercritical water in a first order reaction with E-a = 150 kJ mol(-1) According to thermodynamic calculations, the main products are SiO2, CH4 and H-2 The formation of carbon is also predicted. Films of amorphous and graphitic carbon have been observed, but only small areas of the fibres were coated with carbon. Amorphous silica, quartz, cristobalite and keatite were deposited on the surface of fibres in larger quantities