A solar photocatalytic cascade reactor was constructed to study the photocatalytic oxidation of benzoic acid in water under various experimental and weather conditions at HKUST. Nine stainless steel plates coated with TiO 2 catalyst were arranged in a cascade configuration in the reactor. Photolytic degradation and adsorption were confirmed to be insignificant total organic carbon (TOC) removal mechanisms. A turbulent flow pattern and, hence, improved mixing in the liquid film were achieved due to the unique cascade design of the reactor. The photoinduced consumption of oxygen during reactions was demonstrated in a sample experiment. The proposed rate equations provided good fits to 90 data points from 17 experiments. The regression results showed that the TOC removal rates averaged over 30 min intervals did not illustrate significant dependence on TOC 0 and that I mean was more important in affecting the photocatalytic process within the ranges of the data examined. The percentage removal of TOC in 7 l of 100 mg/l (or 100 ppm) benzoic acid solutions increased from 30% to 83% by adding 10 ml of hydrogen peroxide solution (30 wt%). Hydrogen peroxide was also shown to enhance the efficiency of the degradation process at elevated temperatures. Ortho-, metaand para-hydroxybenzoic acids were identified by HPLC analysis as the intermediates of benzoic acid during reactions without the addition of hydrogen peroxide solutions.