The formation of Ni1–xZnxO rock salt solid solution is obtained by thermal treatments applied to NiO nanoparticles supported on ZnO micrometric particles. The high vapor pressure of ZnO produces a Zn-rich atmosphere during thermal treatment. The Zn ions tend to be adsorbed by the highly reactive NiO nanoparticles. When heated to temperatures high enough, over 500 °C, the Zn ions react with the NiO nanoparticles forming the rock salt material. By variation of the treatment temperature, the composition of this rock salt structure can be varied through the whole solubility range.