The solid-liquid equilibria of the H2O-Co(NO3)(2)-Ni(NO3)(2) ternary system were studied at 248.15 K and 258.15 K, using a synthetic method based on conductivity measurements. The obtained results were complemented by chemical and powder X-ray diffraction analyses. The solid phases are ice and two large substitutional solid solutions: Co(1-sigma)Ni sigma(NO3)(2)center dot 6H(2)O, which has a monoclinic structure, and Ni(1-omega)Co omega(NO3)(2)center dot 6H(2)O, which has a triclinic structure. The formation of a Ni(1-epsilon)Co epsilon(NO3)(2)center dot 9H(2)O solid solution is also highly probable. The ice liquidus curves were modelled in a truly remarkable way, using the quasi-ideal model taking into account ionic dissociation and solvation. The liquidus of the solid solutions were analysed without regard to cation solvation, as a first approximation. Linear behaviour was observed in all cases. On the basis of simple, representative equations, the experimental values were evaluated critically, refined and extrapolated. (C) 2016 Elsevier Ltd.