The influence of X-rays of different wavelength on the degree of structural quality of K(Cl, Br) crystals of mixed composition grown from aqueous solutions is considered. It is found by the methods of chemical etching, X-ray analysis, pycnometric density, and atomic-force microscopy that X-ray irradiation leads to the formation of crystals with a more perfect substructure and surface morphology and a decrease of the dislocation density and concentration of pores and cavities containing the mother liquor. It is shown that X-ray irradiation can promote preferential incorporation of less electronegative components into the crystal lattice of a solid solution.