The mechanical and interfacial properties of the mullite matrix and alumina-silica matrix composites reported in a previous publication [1], are related to the processing parameters. The flexural strengths were in the range of 428-737 MPa, flexural moduli from 82-214 GPa, work of fracture values from 293-482 KJ m(-2) and ultimate failure strains from 0.16-0.45%. The interfacial strengths were determined by the fibre push-in test and compared with fracture behaviour. The formation of different interfaces in the composites and the influence of the density of thermally-induced cracks and porosity on flexural modulus is discussed.