Electron optical techniques have been used to study the hydration of tricalcium aluminate in the presence of gypsum at various pH levels. Calcium aluminosulphate hydrates are precipitated, which have compositional variations and structures that range from highly disordered to highly ordered. Results derived from solubility measurements are able to account for the metastability of ettringite with respect to calcium aluminate monosulphate hydrate at pH more than about 12.8. It is shown that the increased surface area of the precipitates formed at high pH is not sufficient to account for the observed increase in solubility. Consideration of the crystal structure of ettringite and the monosulphate hydrate suggests a further proposal for the type of defect occurring in the ettringite structure at high pH.