Sc3+ and dual Sc3+-Lu3+-doped alpha-SiAlON compositions were sintered by hot pressing, and the formation behavior, microstructure, and mechanical properties were assessed. It was found that the small cation Sc3+ could not be accommodated into the alpha-SiAlON structure alone. The addition of Lu2O3 in the composition induces the Sc3+ cation to enter the alpha-SiAlON structure, and leads to the production of alpha-SiAlON with an elongated-grain microstructure. Transmission electron microscopy analysis shows that alpha-SiAlON grains always contain an alpha-Si3N4 core, implicating heterogeneous nucleation to be in present in a mixed lutetium/scandium-doped alpha-SiAlON system.