This study describes an experimental method for the evaluation of the rate of absorbed photons in an aqueous TiO2 slurry reactor. This method and the use of a macroscopic radiation balance allow for the estimation of extinction coefficients of the dispersed medium. With this end, the effects of the radiation wavelength on the extinction coefficients of TiO2 dispersions are established. The photoconversion of phenol as a model pollutant allows the rate of photodegradation and the effect of the catalyst concentration on this rate to be assessed. In addition, the effects of several other operating parameters on both the phenol photodegradation and the quantum efficiency are studied in detail. It is shown that there is good agreement between parameters estimated using the proposed reaction rate model and data from photoconversion and light transmission experiments.