Densification and stabilization of alpha-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 alpha-structure could be accommodate Ce3+ or Nd3+ cations. In Sr- and La- systems, however, N-(La3Si8O4N11) and S-(similar to SrO1.3Al(2)O(3)0.7Si(2)N(2)O) phases were found to be more stable rather than alpha-sialon. In the case of multi-cations, alpha-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 alpha-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.