Decontamination of opaque fluids using photocatalysts and near Ultraviolet (UV) irradiation involves major technical challenges. This study considers a thin TiO2 layer placed in a new Chemical Reactor Engineering Centre (CREC)-photoreactor cell. This new photoreactor cell is used for the photocatalytic degradation of malic and malonic acids, typical apple juice components. Conversion of organic species can only proceed through the "dark side" of the TiO2 layer, which is in direct contact with the fluid. Under the selected operating conditions both external mass-transfer limitations and photolysis are found to be negligible. Macroscopic radiation balance shows that 92% of near UV radiation is absorbed by the 'back side" of the TiO2-film. Photocatalytic degradation experiments with 10, 20, 30, and 40 ppm malic acid initial concentrations, show that malonic acid is a main intermediate. Complete malic acid conversion occurs after 5-8 h of irradiation. Kinetic modeling of malic and malonic acid photodegradation with kinetic parameter estimation is performed using both an "in series" and an "in series-parallel" reaction networks. The "in series-parallel" reaction network displays better ability for predicting CO2 formation, showing maximum quantum yields of 14.2%. Given that in the CREC-photoreactor cell with a thin TiO2-film, photocatalysis can only proceed via the transfer of mobile "h(+)" sites from the irradiated side to the "dark side', this study demonstrates the significance of this step on the overall photocatalysis mechanism. (C) 2014 American Institute of Chemical Engineers