alpha-sialon starting compositions with m = 1 and n = 1.7 have been densified by either hot-pressing or pressureless sintering using Y2O3 and Ln(2)O(3) additions where Ln is neodymium, samarium, dysprosium or ytterbium. The resulting materials have been heat treated at 1450 degrees C to crystallize the grain-boundary liquid into crystalline oxynitride phases. The effect of sintering additive on the design of final properties has been studied and Dy2O3 was found to be the best sintering additive to obtain desirable properties for alpha-sialon and also alpha-beta sialon materials.