A kinetic model to represent the time evolution of a photocatalytic reaction is presented. It can be applied for reacting systems having the following characteristics : (i) the catalyst is a suspension (in water) of fine particles of titanium dioxide, (ii) the substrate is a hydrocarbon compound amenable to hydroxyl radical attack for the initiation of an oxidative reaction, (iii) the catalyst activation is produced by radiation energy in the near ultraviolet range (300 nm less than or equal to lambda less than or equal to 400 nm), (iv) oxygen is always present in the reacting medium, and (v) there are no mass transport limitations. The model is based on Turchi and Ollis (J. Catal. 122, 178 (1990)) proposed reaction sequence. The described formulation incorporates a precise and detailed evaluation of the radiation energy absorption effects. With this purpose the kinetic model is used in conjunction with a specially developed experimental device (Cabrera et al., Ind. Eng. Chem. Res. 33, 3031 (1994); Alfano et al., Ind. Eng. Chem. Res. 33, 488 (1995)) for which a one-dimensional-one-directional radiation and reactor model has been derived and validated. The model provides an explicit formulation for the radiation-absorbed effects that comprises in a single equation the cases of low and high irradiating conditions. The whole kinetic model development has been applied to the photodegradation of trichloroethylene in water, and it is the subject of a companion paper. Using this approach, reliable intrinsic kinetic data can be obtained.