The influence of incident light intensity and of some physical and chemical parameters on the absorbed and backward reflected photon how and on the reactivity of a heterogeneous photocatalytic system was investigated. The oxidation of phenol in aqueous solution in the presence of polycrystalline TiO2 powder was used as a test reaction. Home-prepared TiO2 (anatase) specimens, subjected to various thermal treatments, were used for the experimental runs performed in a batch photoreactor directly irradiated. The physical parameters investigated were the surface area and the size of particles, while the chemical parameters studied were the initial pH of the suspension and the presence in the reacting medium of additives affecting the photoreactivity such as Cl- and H2O2. It was found that the ratio between backward reflected and incident photon flow does not depend on the used operative variables, while the apparent Napierian extinctance coefficient, which is a parameter related to the photon absorption rate, depends only on the particle size and decreases as the particle size increases. The photoreaction quantum yield, defined as the ratio between the phenol reaction rate and the photon absorption rate, was determined for the various experimental conditions used in this work. The results showed that at equivalent conditions the quantum yield is an intrinsic feature of the semiconductor material; the quantum yield varies only by varying the reaction rate which, in our case, was affected only by the initial pH of the suspension and by the presence of Cl- and H2O2.