Densification and stabilization of ct-sialon ceramics have been investigated by using oxides of large cations (Sr2+, La3+, Ce3+, Nd3+) and their equimolar mixtures with calcium or ytterbium as a principal cation. All multi-cation and single cation (only Ce3, Nd3+) doped samples could be densified nearly full densities even by pressureless sintering. X-ray diffraction on the samples densified with single cations revealed that a- structure could be accommodate Ce3+ or Nd3+ cations. In Sr- and La- systems, however, N-(La3Si8O4N11) and S- (∼ SrO1.3Al2O30.7Si2N2O) phases were found to be more stable rather than cc-sialon. In the case of multi-cations, cc-sialon was observed as the only matrix phase. TEM, SEM, EDX and XRD measurements proved that Sr2+, La3+, Ce3+, Nd3+ cations could be accommodated into α-sialon structure with the presence of calcium or ytterbium cations. SEM and TEM studies on HIPed samples revealed that almost glass free grain boundaries can be obtained by careful design of starting compositions which resulted in high optical transparency. cc-sialons with an elongated grain morphology have been developed in various sialon systems in oxygen and aluminium rich compositions. This type of morphology is more pronounced in oxides of large cation size rare earths (Ce3+ and Nd3+) together with Ca2+. Because of this, fracture toughness of the material has increased due to crack deflection,crack bridging and grain pull-out mechanisms. © 1999 Trans Tech Publications.