A revision of the authors＇ work on suspended solid photocatalytic reactors is presented. Thus, heterogeneous reactions in aqueous media, involving the presence of both fine particles of titanium dioxide and UV radiation are analyzed. Under these conditions, light scattering is a complex phenomenon that must be included in the description of the system performance. Suspended solid photocatalytic reactors are just a family of the well-known slurry reactors for which considerable progress has been reported in the reaction engineering literature. However, ab initio design methods for these particular - light activated - reactors are not known. Previous developed methods should be readily applicable if: (1) the photocatalytic activation is properly described, (2) an intrinsic kinetic expression can be developed, and (3) the scalar field of radiation intensities inside reaction spaces of different geometries can also be known. With this purpose, in order to model the radiation field and also the initiation step, the radiative transfer equation (RTE) for heterogeneous media must be solved. Solution of the RTE in photocatalytic reactors implies the knowledge of different system properties that are not usually known. New methods for obtaining this information are described and typical results are presented. Scale up procedures, derived from the application of first principles, cannot be applied if intrinsic reaction kinetic data are not available. A key point in developing these reaction rate expressions is the knowledge of the radiation absorption by the catalytic particles. The required model is also presented. Furthermore, a simple reactor design and special operating conditions for appropriate laboratory experiments were developed that can be used to produce this type of data reducing to a minimum the difficulties associated with the analysis of composition vs. time information. In this review, the photocatalytic oxidation of trichloroethylene is used as a model reaction. Finally, the description of the radiation field distribution in a typical flat plate, photocatalytic solar reactor simulator illustrates the method that should be used for scaling-up purposes. With this objective the photon absorption rate was obtained as a function of position and the developed model was verified with careful experimental measurements. (C) 2000 Elsevier Science B.V. All rights reserved.