Abstract The concentration and the thermal stability of paramagnetic defects induced by different treatments in ZrO 2 -12%Y 2 O 3 and ZrO 2 -24%Y 2 O 3 are analyzed by studying their EPR signals. The spectroscopic features of the EPR spectra show that the same types of centers are created in X-ray or UV-light irradiated samples and in samples treated in reducing atmosphere at high temperature. The center density depends, in X-ray and UV-light irradiated samples, only on the irradiation dose, and the defect stability is the same in all crystals, independently on the Y 2 O 3 concentration. After reduction of the same samples, the center stability is higher and the center density is different in crystals with different Y 2 O 3 content.