Amorphous precursors of the ZrO2-Fe2O3 system at the ZrO2-rich side of the concentration range were prepared by co-precipitation of the corresponding nitrates from aqueous solutions. The thermal behavior of the amorphous samples obtained was investigated using differential thermal analysis. It was found that amorphous precursors with Fe2O3 content up to 30 mol% are single co-gels. In situ phase development during the calcination of the samples at low air pressure (similar to 4, 10(-3) Pa) was investigated using X-ray diffraction at high temperature. The results were compared with the results of phase analysis after calcination and cooling in the presence of air at atmospheric pressure (similar to 10(5) Pa). The phase compositions of the samples at room temperature were determined using X-ray diffraction and laser Raman spectroscopy. The incorporation of Fe3+ cations partially stabilized cubic ZrO2 during the calcination in the presence of air at atmospheric pressure, but destabilized this polymorph during calcination at very low pressure. The observed differences in the phase development were attributed to the influence of oxygen vacancies introduced during calcination at very low pressure. The solubility of Fe2O, in ZrO2 also depended on the calcination procedure. During calcination at atmospheric pressure, the solubility limits of Fe2O3 in ZrO2 decreased from more than 30 mol% at 600 degrees C to similar to 2mol% at 1100 degrees C. On the other hand, the solubility of Fe2O3 in ZrO2 was shown to be significantly higher during calcination at up to 1200 degrees C at very low pressure.