The isothermal solubility curves of the H2OCo(NO3)(2)Ni(NO3)(2) ternary system were studied at 273.15 and 303.15 K, using a synthetic method based on conductivity measurements. The analysis of the equilibrium phases composition was conducted by UVvisible spectroscopy. The solid phases were first characterized by powder X-ray diffraction, which showed the formation of two substitutional solid solutions: Co(1-sigma)Ni sigma(NO3)(2)center dot 6H(2)O, which has a monoclinic structure, and Ni(1-sigma)Co sigma(NO3)(2)center dot 6H(2)O, which has a triclinic structure. Then the evolution of the lattice parameters of the crystals at 303.15 K, depending on the solid solution composition, was studied by single-crystal X-ray diffraction. The shrinkage of the monoclinic lattice increases with the increase of the nickel ions in the structure. The stability area of this solid solution is very wide as more than 7 cobalt ions in 10 could be replaced by nickel ions. However, the variation of the lattice parameters of the triclinic solid solutions remains limited, although a small shift of the diffraction peaks is observed. This is consistent with a limited composition area of the triclinic solid solution, where less than 1 nickel ion in 10 could be replaced by cobalt ions.