The photocatalytic activity of TiO2 is the result of an interplay between a considerable number of parameters, e.g., phase composition, electronic structure, particle size, exposed surface area, degree of aggregation, mobility of charge carriers, presence of impurities, amount and kind of defects, adsorption of molecules from gas or aqueous phase, lateral interactions between adsorbed species, nature of solvent. etc. Furthermore, these parameters can be broadly subdivided into those that are intrinsic to the photocatalytic material, and those that are extrinsic being influenced by the surrounding environment and conditions. The specific function and influence of a given feature for the photocatalytic performance of a TiO2 sample is difficult to characterize since many of the before-mentioned parameters are strongly coupled. For example, while the degree of aggregation could be inherent to a given material, it is also simultaneously influenced by pH. The degree of aggregation can then influence adsorption of molecules, light scattering and photon adsorption, charge carrier dynamics etc. The plurality of variables driving the nature of the photocatalytic activity, presents a challenge when trying to understand the kinetics and mechanisms underlying photocatalytic processes. It is of primary importance to develop a method to understand and control these properties (or at least some of them). In this paper, we also discuss the relevance of quantum-integrated systems in which the local environment where the molecule is adsorbed is different from the "lonely" photocatalyst or the molecule in solution, and could be treated as a whole. (C) 2010 Elsevier B.V. All rights reserved.