When 3CaO.Al2O3.6H2O (i.e. C3AH6) was placed in contact with a solution of Ca(OH)2 labeled with Ca45 and maintained at 25 °C, there was very little change in either the activity or composition of the liquid. These observations, when taken together, indicate that the compound is probably stable in the system used. On the other hand, when either C3AH6 or 3CaO.Al2O3.CaSO4.12H2O (i.e. C3AΣH12) was added to a mixed Ca(OH)2–CaSO4 solution, the composition of the liquid varied in a manner consistent with the formation of 3CaO.Al2O3. 3CaSO4.31H2O (i.e. C3AΣ3H31). The activity also changed markedly, and a detailed consideration of the relations between activity and composition enabled the reaction mechanisms to be determined. In further experiments, the reactions of C3AH6 and C3AΣH12 to form C3AΣ3H31 were shown to proceed also at 70° and 90 °C. The consistently greater ultimate stability of C3AΣ3H31 over C3AΣH12 between 25° and 90 °C is to be contrasted with the fact, observed in the course of preparing the C3AΣH12, that when solutions of A12(SO4)3, CaSO4, and Ca(OH)2 are mixed the immediate precipitate is C3AΣ3H31 at room temperature, but C3AΣH12 at boiling temperature.