Knowledge of the optical properties of photocatalytic suspensions is vital for a correct comparison of their energetic efficiency. In this work, the determination of both absorption and scattering. coefficients of aqueous suspensions of commercial TiO2 powders irradiated by monochromatic light was carried out by measuring only one quantity-the transmitted photon flow-as a function of the catalyst mass and by applying an asymptotic form of the Kubelka-Munk solution of the radiative transfer equation. Applying a nonlinear fitting procedure the evaluation of the actual values of absorption and scattering coefficients was carried out. The limit for optically thick media of the Kubelka-Munk equation is shown to assume the form of a standard Lambert-Beer absorption formula. The determination of absorbed photon flow was then used to determine the quantum yield value for the phenol photocatalytic oxidation reaction. (c) 2006 American Institute of Chemical Engineers.