Sialon ceramics based on the composition of M2S10Al2O4N14 (M = Sr or Ba) have been sintered, at 1600-1750degreesC, to near-monophase ceramics. Sr loss during sintering results in higher residual silicate phase than in the Ba-stabilised form in which the measured S-phase composition is Ba2Si12-xAlxO2-xN16-x, with x = 2 +/- 0.2. The S-phase crystallises in elongated platelet morphology with preferred growth parallel to the orthorhombic c axis and primary facet planes parallel to (100) and (010). NMR spectroscopy of the Sr-S phase confirms the mixed SiN4/SiN3O tetrahedral coordination suggested from earlier x-ray studies. The Ba-S phase ceramic has an acoustically-measured Young modulus of >220 GPa, a hardness of 13.5 GPa and a fracture toughness of 4.5-5 MPa. rootm.