A study of the quantum efficiency of the photocatalytic degradation of 4-chlorophenol (4-CP) in a TiO2 slurry reactor is presented. Also, in order to assess the final conversion of the pollutant into CO2, the efficiency of the mineralization process is defined and evaluated. To calculate these efficiencies, the radiation absorbed by the heterogeneous reacting system needs to be determined. A one-dimensional, one-directional radiation field model is proposed to compute the photon absorption inside the slurry photocatalytic reactor. Degradation rates and quantum efficiencies are analyzed considering the influence of three operating variables: pH, catalyst loading, and irradiation intensity. Important dependencies on these variables were found. The best efficiencies were obtained in acidic conditions, at the highest employed catalyst concentration (1.0 x 10(-3) g cm(-3)) and at the lowest level of irradiation.