AbstractSeveral modifications of the ‘hexagonal’ tetracalcium and dicalcium aluminate hydrates have been obtained, some of which have not been described previously. Only a newly established 4CaO,Al2O3, 19H2O hydrate, and an α‐form and a hitherto undescribed β‐form of a 2CaO,Al2O3,8H2O hydrate have been found to occur in the aqueous system at 25°. Other modifications are produced on drying under various conditions. The 4CaO,Al2O3,19H2O hydrate, characterized in the X‐ray patterns by a longest basal spacing of 10·6Å, is dehydrated relatively easily to give a mixture of α‐and β‐forms of a 13H2O hydrate, characterized by longest basal spacings of 8·2Å and 7·9Å, respectively. There is little difference in stability between the two forms and it appears to be a matter of chance as to the relative amount of each which may be formed. Two other tetracalcium aluminate hydrates have been obtained with water contents of 11H2O and 7H2O, both of which give longest basal spacings of 7·4Å. The α‐ and β‐forms of 2CaO,Al2O3,8H2 are characterized by longest basal spacings of 10·7Å and 10·4Å respectively. The α‐form appears to be more stable than the β‐form. Three other states of hydration of the dicalcium compound exist, having water contents of 7·5H2O, 5H2O and 4H2O, and giving longest basal spacings of 10·6Å, 8·7Å, and 7·4Å, respectively. With various preparations of the compound 3CaO,Al2O3,CaSO4,aq., basal spacings of 10·3Å, 9·6Å, 8·9Å, 8·2Å, and 8·0Å have been observed, but these modifications await further study.