Abstract: A silicon aluminium oxynitride product having the crystal lattice structure of silicon oxynitride but of expanded cell dimensions is produced by sintering a powder mixture of 50 to 60 wt % of Si.sub.3 N.sub.4, 22 to 35 wt % SiO.sub.2, 1 to 20 wt % Al.sub.2 O.sub.3 and 1 to 15 wt % of at least one oxide of Y, Li, Mg, Ca, Sc, Ce or another rare earth element, the molar ratio of SiO.sub.2 to Si.sub.3 N.sub.4 being greater than 1:1 and the percentages being based on the total weight of the ingredients. Sintering is effected at 1600.degree. to 1900.degree. C. in a non-oxidizing atmosphere so that the Al.sub.2 O.sub.3, said at least one oxide and part of the SiO.sub.2 in the mixture react to form a liquid phase from which silicon aluminium oxynitride having an expanded silicon oxynitride crystal lattice is crystallized. The sintered material is cooled to form a product containing said silicon aluminium oxynitride and at least one grain boundary phase containing silicon and the metal(s) of said at least one oxide.