NiAl2O4,NiFe2O4, and their solid solution Ni(FezAl1−z)2O4 have been studied using high temperature oxide melt solution calorimetry in molten 2PbO·B2O3 at 973 K. X-ray absorption near edge structure (XANES) measurements and Mössbauer spectroscopy investigation of the cation distribution showed that the Ni fraction in octahedral sites increases with increasing iron content. Despite the zero heat of mixing, the solid solution is not thermodynamically ideal in the sense of Raoult's law. The entropies of mixing are similar to those for a solid solution of two inverse spinels and the activities are approximated as the square of the mole fractions. The stability of the solid solutions relative to oxide components (NiO,Al2O3,Fe2O3) increases with increasing iron content. The solid solution is a suitable waste form for nickel from industrial processing with higher iron content potentially beneficial to its stability.