Nanoporous zirconia is used in gas sensors and as a membrane in high-temperature fuel cells. In the present work, positron annihilation spectroscopy and transmission electron microscopy (TEM) were performed on pure zirconia-sintered nanopowders, to determine the porosity. The ortho-positronium annihilation parameter R of zirconia samples, treated at 800ºC and 700ºC and annealed in oxygen–nitrogen atmosphere with different O2 contents, were obtained. The photoluminescence, positron annihilation spectroscopy, and TEM studies show presence of defects in all samples. Furthermore, the positron annihilation studies indicate a presence of large free volumes (of the order of few atomic units, at least), open towards the nanocrystals surface what was confi rmed by TEM observations which detected a few types of defects such as voids within ≈2–4 interplane distances, stacking faults, terraces and point defects.