The solubilization of excess sludge by the solar photo-Fenton reaction has been investigated for the reduction of excess sludge in the activated sludge process. The solubilization kinetics depended on the dosages of the Fenton reagents, Fe and H(2)O(2). Increases of initial Fe and H(2)O(2) concentrations in their ranges studied in this work continuously enhanced the sludge solubilization. Cell lysis by the photo-Fenton reaction caused the increase in dissolved chemical oxygen demand (COD) in the first step of sludge solubilization. The further oxidative decomposition of the discharged organic compounds by the photo-Fenton reaction led to the decrease in the dissolved COD as the second step of sludge solubilization. The increase of dissolved COD in the first step of sludge solubilization and the consumption of H(2)O(2) could be described by the pseudo-zero order kinetics based on the accumulated light energy. About 40% reduction of mixed-liquor suspended solids (MLSS) by the solar photo-Fenton reaction was found. It was found that solar light used as a light energy source instead of costly and hazardous artificial UV light was very effective. The dissolved COD for solar photo-Fenton reaction increased faster and by 1.5 times as compared with that by artificial UV light.