Samples of zirconium oxide doped with titanium oxide or cadmium oxide, prepared as a divided state, were examined to investigate the effect of the foreign ions on the zirconia phase transition. Thermal treatments (1173 K in H-2 for TiO2-ZrO2; 1073 K in air for CdO-ZrO2) caused the incorporation of titanium or cadmium in solid solution in the zirconia structure (tetragonal or cubic metastable phases). The solid solution formation hindered the retragonal --> monoclinic transformation that takes place in undoped ZrO2 during the cooling step.