A new double salt, RbBr . NiBr2. 6H(2)O, crystallizing as the only possible double salt at T = 298.15 K from aqueous rubidium-nickel bromide solutions was predicted using Pauling rules and the "hard" and "soft" Lewis acids and bases concept. The system (m(1)RbBr + m(2)NiBr(2))(aq), where m denotes molality, was thermodynamically simulated by the Fitter method under the assumption that the solubility diagram consists only of three branches, namely RbBr, RbBr . NiBr2. 6H(2)O, and NiBr2. 6H(2)O. The theoretical result was experimentally proved at T = 298.15 K by the physicochemical analysis method, and the formation of the new double salt RbBr . NiBr2. 6H(2)O was established. The double salt crystallizes in a monoclinic crystal system, space group C2/c, a = 0.9572(2)nm, b = 0.9759(2) nm, c = 1.3666(2)nm, beta = 90.12(2)degrees, V = 1.2766(4)nm(3), unit cell Z = 4, unit cell density D-x= 2.560g/cm(-3). The binary and ternary ion interaction parameters were calculated and the solubility isotherm was plotted. The standard molar Gibbs energy of the synthesis reaction Delta(r)G(m)degrees, of the double salt RbBr . NiBr2. 6H(2)O from the corresponding simple salts RbBr and NiBr2. 6H(2)O, as well as the standard molar Gibbs energy of formation Delta(r)G(m)degrees of the simple and double salts were calculated.