The (001) surface of tetragonal zirconia, both bare, and fully hydrated or hydroxylated has been theoretically studied. A periodic Hartree-Fock method has been used, by simulating the adsorbent with a thin slab. Pseudopotentials have been employed for core electrons, and a “split-valence” set for valence electrons. Several geometries have been considered for surface water or OH groups. In the two cases, one preferential structure has been individuated, corresponding to a similar adsorption energy (∼ 15 kcal/mol with respect to isolated water molecules); the corresponding electronic structure has been analyzed. The vibrational frequencies of adsorbed OH groups have been calculated; they differ by more than 350 cm−1. Similar differences in OH vibrational frequencies have not been observed in infrared studies of zirconia, which, however, concern the monoclinic phase.