[en] The pH-metric titration, X-ray diffraction method, and thermogravimetric analysis have been used for studying the products of interaction of Rb⁺ and Cs⁺ with crystal zirconium arsenate of the composition of Zr(HAsO₄)₂xH₂O in aqueous solutions at 90 deg C. It is shown that interaction proceeds by the exchange mechanism and gives rise to the formation of isostructural crystal phases of the composition of ZrMH(AsO₄)₂xH₂O (where N--Rb,Cs). The heating of these phases is accompanied by their dehydration, double salts of RbZr₂(AsO₄)₃ and CsZr₂(AsO₄)₃ being the final products of thermal decomposition. The crystal lattice parameters are given