The conversion of Zr(HPO 4) 2 · H 2O into ZrHNa(PO 4) 2 · 5H 2O, followed by regeneration with dilute solutions of HCl, yields a mixture of zirconium phosphate materials with various degrees of hydration and different interlayer distances. The relative amounts of the hydrated phases in the mixture were strongly dependent on the method of preparation of the employed Zr(HPO 4) 2 · H 2O. Under appropriate experimental conditions, 100% of a phase having an interlayer distance of 10.4 Å (already reported as Zr(HPO 4) 2 · 8H 2O and called ϑ-ZP) has been obtained. The stability of the various hydrated phases in HCl solutions of different concentrations was investigated and a valuation of their water content was attempted. The ion exchange properties of ϑ-ZP for alkali, alkaline earth and some divalent transition metal ions have been investigated in detail and a comparison with the ion exchange properties of Zr(HPO 4) 2 · H 2O is made. ϑ-ZP and other highly hydrated dihydrogen phases, owing to their expanded interlayer distance, can easily exchange several cations which are not, or only slowly, exchanged on the monohydrate dihydrogen form due to steric hindrance. This fact considerably enlarges the practical use of crystalline zirconium phosphate.