The phase and microstructural evolution of multi-cation Sm–Ca–α-sialon ceramics was investigated. Six samples were prepared, ranging from a pure Sm-sialon to a pure Ca-sialon, with calcium replacing samarium in 20 eq% increments, thus maintaining an equivalent design composition in all samples. After pressureless sintering at 1820 °C for 2 h, all samples were subsequently heat treated up to 192 h at 1450 and 1300 °C. The amount of grain boundary glass in the samples after sintering was observed to decrease with increasing calcium levels. A M′ss or M′ss-gehlenite solid solution was observed to form during the 1450 °C heat treatment of all Sm-containing samples, and this phase forms in clusters in the high-Sm samples. The thermal stability of the α-sialon phase was improved in the multi-cation systems. Heat treatment at 1300 °C produces SmAlO3 in the high-Sm samples, a M′ss-gehlenite solid solution in the high-Ca samples, and a Sm–Ca–apatite phase in some intermediate samples.