A series of SiC fibre-reinforced Si3N4-AlN-Al2O3-Y2O3 matrix composites with different matrix compositions are fabricated by slurry infiltration followed by hot pressing at 1600degreesC for 30 min. The diffusion of yttrium and aluminium into fibres is apparent during the high temperature processing. All the as-processed composites show fracture with fibre pull-out. After heat treatment in air at 1000degreesC for 60 min, composites with minimal Y2O3 and Al2O3 in the matrix composition demonstrate the fracture behaviour with most extensive fibre pull-out. Composites with the highest aluminium and yttrium oxide content form an yttrium-aluminium-garnet phase and an aluminosilicate glassy phase. The latter phase provides an oxygen diffusion path, resulting in the removal of the carbon-rich interphase by oxidation. This results in catastrophic fracture without fibre pull-out after heat treatment of the composite in air.