In the past decade an increasing interest in photocatalytic kinetic studies has been observed, particularly related to promising remediation processes for air and water pollution. Normally, the reacting system includes a suspended-solid semiconductor and one or two fluid phases. One of the major problems lies in the difficulties associated with the proper evaluation of the absorbed radiant energy due to the unavoidable system heterogeneities that produce light scattering. A novel form of reactor, combined with a radiation distribution model, has been used to evaluate the volumetric rate of energy absorption during the photocatalytic oxidation of trichloroethylene in water using a suspension of titanium dioxide. All the required information to solve the radiative transfer equation in a one-dimensional photocatalytic reactor is obtained either from radiation theory or from specially designed experiments. The proposed approach permits a correct description of the radiation field inside the heterogeneous reactor and, consequently, a precise accounting of the absorbed photons. The quantum yield concept is revisited to propose an appropriate and equivalent property for solid photocatalyzed systems. Afterward, the reactor and the model were used to evaluate heterogeneous-system quantum efficiencies.