[en] The ion exchange behaviour of zirconium phosphates possessing different degrees of crystallinity was examined for the NH₄⁺/H⁺ system. In the most crystalline exchangers ammonium ion is initially taken into solid-solution without phase change. This initial uptake is followed by conversion of the solid solution to a fully exchanged ammonium ion phase Zr(NH₄PO₄)₂ .H₂O. The composition range of the solid solution increases with decreasing crystallinity until, in the amorphous exchanger it covers the entire range of ammonium ion uptake. The exchange process is reversible for the amorphous exchanger with K = 7.4 x 10^-5. However, with the more crystalline exchangers a half-exchanged phase, Zr(NH₄)H(PO₄)₂.0.33H₂O, is obtained when H⁺ replaces NH₄⁺. These results differ significantly from other published data. (author)