Summary: The existence and relationships of hydrated forms both to one another and to anhydrous CaSO₄ have been studied by X-ray methods. Detailed study of the hemi-hydrate reveals its true cell to be monoclinic and does not confirm the previously suggested polymorphism.

Dehydration studies of the dihydrate and hemi-hydrate are reported; cyclic dehydration and hydration of single crystals of the latter show a variable temperature interval between the formation of γ-CaSO₄ and the reconstitution of the hemihydrate at approximately constant humidity. An explanation of this behaviour is offered, and the structural implications of the results of the dehydration studies are discussed.