The concentration dependence of the decomposition temperature of metastable rs-Мe 1-x Zn x O solid solutions (T d vs x) at ambient pressure for all investigated systems is shown in Fig. . It is easy to see that the cubic solid solutions of the NiO-ZnO system have the highest phase stability, and they also have the widest range of concentration stability . Second and third in terms of thermal stability are the cubic solid solutions of the CoO-ZnO and MnO-ZnO systems, respectively. Finally, the cubic solid solutions of the FeO-ZnO system have the lowest phase stability, probably due to the ease of the redox disproportionation processes (3 Fe 2+ → 2 Fe 3+ + Fe 0 ) observed already at relatively low temperatures.