The present study focuses on the determination of intrinsic kinetic data for photocatalytic reactions. Several powders of titania: commercial (Degussa P25, Aldrich Anatase) and that prepared by sol-gel synthesis were evaluated for the UV-induced heterogeneous degradation of 2,4,6-trichlorophenol. It was demonstrated that kinetic data obtained from commonly used photocatalytic reactors (such as a liquid-phase annular reactor) do not correspond to the intrinsic activity of catalysts because of strong nonuniformity of the internal radiation field. The latter was determined by the measurement of local values of incident radiation at the inner and outer walls of the reactor. A photoreactor with a variable reaction zone in both thickness and length of the annulus was constructed. It was shown that by reducing the reaction volume one can minimize the radiant transfer resistances and reduce the effect of the nonuniformity of the radiation field in both radial and axial directions. As a result of the gradual reduction of the reaction zone size, the overall quantum yield was found to increase and approach a plateau for the smallest volume (annulus of 0.15 cm thickness and 2 cm length). Therefore, a critical volume exists for photocatalytic systems, which can yield intrinsic properties. The quantum yields obtained in this volume had comparable values for different photocatalysts, although their apparent reaction rates were quite different.