Powders of polymorphic zirconia contain crystallites incorporating monoclinic and tetragonal domains. Thermal activation in high vacuum results in a significant amount of highly reactive radical surface centres (O^–, Zr³⁺) mainly located in tetragonal surface sites. Their exposure to molecular H₂ leads uniquely to homolytic dissociation processes clearly evidenced by IR spectroscopy. These processes give rise to an irreversible formation of multiple-coordinated surface OH groups on tetragonal surface sites (3668 cm^–1) and to a reversible formation of simple (ZrH; 1565 cm^–1) and bridged hydrides (Zr₂H; 1360 cm^–1). Simple hydrides are very reactive towards any kind of oxygen species (O₂, O). Their reaction with physisorbed O₂ leads to irreversible formation of multiple-coordinated OH groups and O₂^.– radical anions [ν(O—O)^.–; 1118 cm^–1] evidenced by ¹⁸O₂ isotopic experiments. The appearance of these OH groups necessarily implies surface mobility of intermediately existing H atoms on both monoclinic and tetragonal surface domains.