alpha-SiAlONs are commonly produced by liquid phase sintering Of Si3N4 with AlN and Y2O3 as additives. The formation of the alpha-SiAlONs using a mixed oxide (RE2O3), containing yttria and rare-earth oxides, as an alternative additive was investigated. Dense alpha-SiAlONs were obtained by gas-pressure sintering, starting from (alpha-Si3N4 and AIN-Y2O3 or AIN-RE2O3 as additives. The mixed oxide powder RE2O3 was characterized by means of high-resolution synchrotron X-ray diffraction and compared to Y2O3. The X-ray diffraction analysis of the mixed oxide shows a pattern indicating a true solid solution formation. The Rietveld refinement of the crystal structure of the sintered alpha-SiAlON using AlN-RE2O3 as additive revealed a similar crystal structure to the alpha-SiAlON using AlNRE2O3 as additive. The comparison of the microstructures of the both alpha-SiAlONs produced using pure Y2O3 or RE2O3, revealed similar grain sizes of about 4.5 mu m with aspect ratios of about 5. Both materials show also similar 12 mechanical properties, with hardness of 18.5 GPa and fracture toughness of 5 MPa m(1/2). It could be, thus, demonstrated that pure Y2O3 can be substituted by the rare-earth solid solution, RE2O3, in the formation of alpha-SiAlONs, presenting similar microstructural and mechanical properties. (c) 2005 Elsevier Ltd. All rights reserved.