α-SiAlONs are commonly produced by liquid phase sintering of Si₃N₄ with AlN and Y₂O₃ as additives. The formation of the α-SiAlONs using a mixed oxide (RE₂O₃), containing yttria and rare-earth oxides, as an alternative additive was investigated. Dense α-SiAlONs were obtained by gas-pressure sintering, starting from α-Si₃N₄ and AlN–Y₂O₃ or AlN–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 α-SiAlON using AlN–RE₂O₃ as additive revealed a similar crystal structure to the α-SiAlON using AlN–RE₂O₃ as additive. The comparison of the microstructures of the both α-SiAlONs produced using pure Y₂O₃ or RE₂O₃, revealed similar grain sizes of about 4.5μm with aspect ratios of about 5. Both materials show also similar mechanical properties, with hardness of 18.5GPa and fracture toughness of 5MPam^1/2. It could be, thus, demonstrated that pure Y₂O₃ can be substituted by the rare-earth solid solution, RE₂O₃, in the formation of α-SiAlONs, presenting similar microstructural and mechanical properties.