The crystal structure of fully hydrated synthetic zeolite type A has been described. Co2+, Ni2+, Cu2+ and Mn2+ ions have been exchanged to different extents (from 16.6% up to 81. 6%) into zeolite A and their zeolitic water contents have been measured thermogravimetrically. The gravimetric studies showed complete loss of water molecules in dynamic vacuum (10(-6) torr) at 623 K. Turnover numbers for H2O increase smoothly with increasing transition metal divalent ions in the zeolite framework. The water content is closely related to the nature and number of the cations incorporated into the zeolite lattice. The sequence of the number of water molecules per unit cell was Ni A> Co A> Cu A> Ma A> Na A. Thus the water content increases with decreasing ionic radius of the cation, from 27 for Na+ (r = 0.98 A-degrees) to 47.8 for Ni2+ (r = 0.69 A-degrees)