Mechanochemical equilibration of the ternary system NaBr/CsBr/H2O was investigated using a ball mill which is a closed system. Products obtained by grinding mixtures with different molar ratios of NaBr, CsBr, and H2O were identified by X-ray diffraction, IR spectroscopy, and thermal analysis. The water content of the different species in the mixture was determined by thermogravimetry. The products of grinding are solid solutions of hydrated NaBr in CsBr, Cs1-x[Na(H2O)]xBr (x ≤ 0.48), the hydrated double salt Cs[Na(H2O)]2Br3, and NaBr·2H2O. The phase diagram of the NaBr/CsBr/H2O system was constructed for that part in which only solids exist in the equilibrium state. From the phase diagram it is obvious that the double salt cannot be crystallized from aqueous solutions.