Three crystalline forms (cubic, orthorhombic I and orthorhombic II) exist in the Na₂O–Ca₃Al₂O₆ solid-solution series at room temperature. A tetragonal form may be obtained by quenching. All the phases have lattices extremely close to the cubic form. The cubic and tetragonal varieties show no polymorphic transformations. The orthorhombic O_I and O_II phases become tetragonal at high temperature. There is complete agreement between the transition points revealed by X-ray diffraction and microscopy. However, only the order–disorder O_II→T transition is observed by μDTA.