The amorphous precursors of the HfO2-Fe2O3 system at the HfO2-rich side of the concentration range were prepared by the co-precipitation of the corresponding salts from aqueous solutions. The thermal behavior of the samples was followed using differential thermal analysis and X-ray diffraction at high-temperature. The differences in the phase developments during the heating of the samples in the presence of air at atmospheric pressure (similar to 10(5) Pa) and low pressure (similar to4 x 10(-3) Pa) were attributed to the influence of the oxygen vacancies introduced during calcination at low pressure. The results of phase analysis showed that Fe3+ ions have both a stabilizing and destabilizing influence on the high-temperature polymorphs of HfO2. The incorporation of 10-30 mol% of Fe2O3 stabilized cubic hafnia during calcination in the presence of air at atmospheric pressure, but further increase of the iron content caused the destabilization of this phase. The destabilizing influence of the Fe3+ ions increased during calcination at low pressure. The oxygen vacancies introduced during the calcination at low pressure partially stabilized the cubic HfO2, but this stabilizing influence significantly decreased in the samples with high iron content. The obtained results indicated that both the stabilizing and destabilizing influences of Fe3+ ions are related to the oxygen vacancies introduced in the HfO2 lattice. The stabilization of the high-temperature polymorphs of HfO2 depended on the amount of oxygen vacancies and their distribution inside the lattice. The solid solubility limits: of Fe2O3 in HfO2 decreased during heating at atmospheric pressure from >30 moL %at 700 degreesC to similar to2 mol% at 1000 degreesC. The solubility of Fe2O3 in HfO2 during the heating at low pressure was higher.