In relation to the synthesis of a ceramic matrix composite reinforced by Si-Ti-C-O fibre (Tyranno), the pyrolysis of Tyranno in alumina powder was investigated at temperatures, T, of 1773-1973 K under Ar or N-2 atmosphere. Using a Tyranno-alumina compact, pyrolytic rates were determined with a thermobalance. The heat-treated fibres were examined by X-ray diffraction (XRD) and scanning electron microscope (SEM) observation. Under N-2 atmosphere and at lower temperatures, pyrolysis of Tyranno was retarded by the formation of a nitrided case. The retardness effect disappeared in the presence of alumina powder. At elevated temperatures, pyrolysis was suppressed by the presence of alumina powder. Alu mina was reduced to be nitrided into AIN by the gas-phase reaction between the pyrolytic gas, CO and N-2. In addition, Tyranno crystallized into beta-SiC and alpha-SiC of type 2H.