A SiAlON ceramic with yttria as a sintering aid has been fabricated and subjected to a 1‐h heat treatment at 1150°C. Microstructural characterization by analytical electron microscopy and diffraction analysis indicates that the residual intergranular phase crystallizes into a metastable hexagonal phase (Y2SiAlO5N) and mullite. The mullite appears as a characteristically shaped precipitate forming with a definite orientation relationship on the (1010) facets of SiAlON grains. The absence of a residual amorphous film between the precipitate and the SiAlON indicates that the precipitation occurred in a heterogeneous manner on the surface of the SiAl