Partial dehydration of NH4Fe, KCr, KAI, and NH4Al sulphate alums in vacuo, and their subsequent exposure to water vapour, have been investigated at 19–23° using dielectric measurements at 4·0 Mc./sec. Initial values of Iµ′ and Iµ″ relate to equilibrium water vapour pressure data, and are higher for crushed samples. Marked decreases in Iµ′ and Iµ″ occur in the early stages of the dehydration of NH4Fe and KCr alums, but not with KAl and NH4Al alums, and the curve of Iµ″ against time for the dehydration of NH4Fe alum shows a secondary maximum. Dehydrated NH4Fe and KAl alums sorb water more readily than the dehydrated KCr and NH4Al alums. Changes in Iµ″ during re-hydration are shown to reflect changes in adsorbed water and are independent of water of hydration. At constant re-hydration pressure, Iµ″ increases linearly with the equilibrium water vapour pressure of the original alum, indicating that adsorbed water is bound to the surface by a mechanism similar to that with which water of crystallisation is held inside the lattice. Changes in Iµ″ during re-hydration of dehydrated NH4Fe alum differ from previous results obtained with Rochelle salt.