In the literature two intrinsic paramagnetic centers have been identified by EPR in pure zirconia: Zr3+ ions, yielding an axial signal (g(parallel to) = 1.958 and g(perpendicular to) = 1.979) and electrons trapped in oxygen vacancies (V (o) over circle). In this study we investigated by quantitative-EPR pure and doped ZrO2 heated in air and redox treated at various temperatures. In pure and in doped zirconia the axial signal has similar spectral features, including the satellite line due to the most intense component of Cr-53 hyperfine structure, and the signal intensity yielded the same redox trend. In doped zirconia the concentration of the species yielding the axial signal was a constant fraction (about 50%) of the total chromium content. In pure zirconia samples, the concentration determined by EPR was also about 50% of the Cr impurity level determined by GFAAS. These findings attribute this signal to the extrinsic center, Cr5+ ion, arising from chromium impurity. In reduced zirconia a signal of Lorentzian shape (g = 2.002; DeltaH(pp) = 4-5 G) arises from the only intrinsic defect detected by EPR in zirconia bulk, namely V (o) over circle center. This signal could easily be confused with another having the same g value, but a different line width (DeltaH(pp) = 0.5-1 G) and line shape that forms in some vacuum-activated zirconia samples and arises from an extrinsic center, carbon paramagnetic impurity.