It is well-known that NiO-CoO solid solutions can easily be formed. However, the formation of Ni3O4-Co3O4 solid solutions has not been recognized. In this article, it is reported that, whereas the calcination of a Ni(NO3)(2) and Co(NO3)(2) mixture with a 1:1 molar ratio at a high temperature above 800 degrees C generated NiO-CoO solid solution, only a Ni3O4-Co3O4 solid solution was observed after calcination at a low temperature of 500 degrees C. Furthermore, if the calcination was carried out in an intermediate temperature range from 600 to 700 degrees C, both NiO-CoO and Ni3O4-Co3O4 solid solutions could be formed. This occurred because Co3O4 can induce the formation of Ni3O4, whereas NiO can stabilize CoO In addition, the lattice parameter of Ni3O4 was predicted using Vegard's law to be 8.2054 angstrom.