Based on the idea that eutectic formation is an useful technology for controlling the melting points of latent heat storage materials over a wide temperature range, precise experimental studies were carried out on the inevitability of eutectic composition of salt hydrate mixtures. Phase diagrams were developed from heating curves of two pseudo-binary systems, Ni(NO3)(2)center dot 6H(2)O-CoCl2 center dot 6H(2)O and NiCl2 center dot 6H(2)O-Co(NO3)(2)center dot 6H(2)O. The melting points of individual hydrates were 53.5, 53.0, 51.5 and 53.5 degrees C, respectively. In the two binary systems, similar simple eutectic formations were recognized under the same conditions of 30 degrees C and equimolar composition. The crystal compositions in the systems changed depending on the binary mixing ratio. At the same time, new crystal phases that were not attributed to either of original salt hexahydrates were also generated, and the contents of the new phases became maximum at the eutectic points. The two eutectics generated were found to have the same crystal composition as each other as analyzed by X-ray powder diffraction patterns. Consequently, it was concluded that the eutectics were formed through the mutually equivalent exchange of dissociated ions. Further, the inevitability of those compositions was determined as the mixing ratio at which the mutually equivalent exchange might be maintained for all dissociated ions.