A crystallographic study by Weissenberg photographs of the hydration and dehydration of hewettite and metahewettite is reported. The formula may be taken as CaO.3V 2 O 5 .nH 2 O, with n varying from 3 to 9, depending on the humidity. It was found that one structure (space group P2/m, Z = 1) persists from 9H 2 O to almost 5H 2 O, at which stage a discontinuous transition to a new structure (A 207, Z = 2) takes place, and these are designated hewettite and metahewettite respectively. The changes are reversible, with the hewettite structure regained after hydration. The relationships between the structures and the Na-analogue of metahewettite are considered.