In tricalcium silicate and its solid solutions six modifications were established: one rhombohedral (R), two monoclinic (MI, MII), and three triclinic (TI, TII, TIII). All these forms are rhombohedral or pseudorhombohedral and extremely similar; the variations in the X‐ray diagrams and the transformation enthalpies are minute. Only three of the five transitions were observed by both X rays and DTA: TI‐TII (600°), TIII–MI (980°), and MI–MII (990°); one was established by DTA (TII–TIII, 920°) and one by X rays only (MII–R, 1050°C). This scheme of polymorphism is compared critically with results of other authors, and the nature of the various displacive transformations is discussed. X‐ray and DTA investigations of solid solutions of Ca3SiO5 with Al2O3 and ZnO established the phase relations as functions of temperature and composition. With Al2O3, two modifications (TI and TII) were stabilized; and with ZnO, five polymorphs (TITII, MI, MII, and R). With both pure C3S and the solid solutions, the results are consistent and have a close bearing on the constitution of alites in portland cement clinker.