Studies involving the TiO₂-assisted photodegradation of organic substances report that the processes are photocatalytic in nature. Yet, no evidence exists confirming such assertions. Previously, we examined the usage of relative photonic efficiencies [N. Serpone, G. Sauve, R. Koch, H. Tahiri, P. Pichat, P. Piccinini, E. Pelizzetti, H. Hidaka, J. Photochem. Photobiol. A: Chem. 94 (1996) 191; N. Serpone, J. Photochem. Photobiol. A: Chem. 104 (1997) 1] and quantum yields Φ [N. Serpone, R. Terzian, D. Lawless, P. Kennepohl, G. Sauve, J. Photochem. Photobiol. A: Chem. 73 (1993) 11] to systematize discrepant claims about process efficiencies. An experimental protocol is now available [N. Serpone, A. Salinaro, Pure Appl. Chem. 71 (1999) 303] to measure true Φ in heterogeneous media. Photoinduced reduction of O₂ and photooxidation of H₂ occurring on oxidized and reduced surfaces of ZrO₂ were recently examined [A.V. Emeline, G.N. Kuzmin, L.L. Basov, N. Serpone, J. Photochem. Photobiol. A: Chem. 174 (2005) 214] to probe the spectral variations of the photoactivity and photo-selectivity of ZrO₂ by determining Φ for the two redox reactions at various wavelengths of photoexcitation (200 < λ < 400 nm). Irradiation of ZrO₂ in the intrinsic absorption region (λ < 260 nm) led predominantly to photoreduction of O₂, whereas photooxidation of H₂ predominated on irradiation in the extrinsic spectral region (260 < λ < 360 nm). A difficult task in heterogeneous catalysis and photocatalysis is determination of the actual number of active sites, an issue that has heretofore been elusive to assess the (photo)catalytic activity of a given material in heterogeneous solid-liquid and solid-gas (photo)catalysis. A kinetic description of the three turnover quantities, viz., turnover number (TON), turnover rate (TOR) and turnover frequency (TOF) has been described [N. Serpone, A. Salinaro, A.V. Emeline, V.K. Ryabchuk, J. Photochem. Photobiol. A: Chem. 130 (2000) 83], concluding that turnover quantities are conceptually distinct, with TON and TOR requiring knowledge of the number of active sites on the (photo)catalyst's surface. Apparently, turnovers depend on the nature of the active state of the catalyst and how it is described. The number of surface-active sites on the ZrO₂ particle surface have been determined quantitatively (∼10¹⁶ active centers) through thermoprogrammed desorption spectroscopy, affording an estimate of TONs for the photooxidation of H₂ (TON > 14.5) and photoreduction of O₂ (TON > 6.6) on ZrO₂ and demonstrating for the first time that a photoreaction occurring on the surface of a metal oxide is truly photocatalytic [A.V. Emeline, A.V. Panasuk, N. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 (2005) 2785]. Photocoloration of a metal oxide such as ZrO₂ (process of photoinduced formation of Zr³⁺, F- and V-type color centers) during a surface photochemical reaction was also used to assess whether a reaction is photocatalytic. Our study on the influence of simple photoreactions involving the photoreduction of O₂, photooxidation of H₂, photooxidation of H₂ by adsorbed O₂, and photoinduced transformation of NH₃ and CO₂ on the photocoloration of ZrO₂ concluded that photoreactions involving NH₃ and CO₂ are non-photocatalytic processes, in contrast to the photooxidation of H₂ which is photocatalytic [A.V. Emeline, G.V. Kataeva, A.V. Panasuk, V.K. Ryabchuk, N.V. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 (2005) 5175]. In this article, we describe the criteria and conditions by which a photoreaction taking place on the surface of a solid can be said to be photocatalytic by considering both a chemical approach and a physical approach.