The photoadsorption of oxygen (photoreduction on electron surface-active centers) and the photoadsorption of hydrogen (photooxidation on hole surface-active centers) as well as the photooxidation of hydrogen in the presence of oxygen were examined over irradiated zirconia (ZrO2) specimens by thermoprogrammed desorption spectroscopy (TPD) and kinetically to assess the states (forms) of oxygen species formed on the surface of zirconia. The three TPD spectral maxima observed inferred three oxygen species of varying activity in the photooxidation of hydrogen. The number of surface-active sites on the zirconia surface were quantitatively estimated (ca. 1 x 10(16) centers), thereby permitting an estimate of the turnover numbers (TON) for the photooxidation of hydrogen (TON > 14.5) and for the photoreduction of oxygen (TON > 6.6). These demonstrate for the first time that a photoreaction occurring on the surface of a metal-oxide photocatalyst is truly photocatalytic.