Duplex Y-, Yb- and Nd-doped α-β-sialon ceramics were fabricated from starting powder compositions within the two-phase α -β area. The microstructures contained high aspect ratio β-sialon grains homogeneously distributed in an α-sialon matrix and only smaller volume fractions of intergranular phases. The morphology of the intergranular regions was determined by the overall chemistry and the doping element. The β /α +β weight ratio as well as the α and β-sialon substitution levels were dependent upon the α stabilizing cation. Sialons with an β /α +β weight ratio of around 0.5 had a favourable combination of high hardness, HV 1 0 around 21 GPa, and indentation fracture toughness, 5.0-5.5 MPa m 1 2 . The toughness of these duplex sialons was however reduced to below 4 MPa m 1 2 after long-term heat treatment at 1450°C. The α stabilizing elements Y and Yb resulted in α-sialon structures which were stable during heat treatment at 1450°C, while the Nd α structures decomposed into Al-substituted melilite, Nd 2 Si 2 AlO 4 N 3 , β-sialon and the 21R sialon polytype at this temperature. The results imply that duplex α-β-sialons stabilized by Y and Yb have a high potential as engineering ceramics.