The photosensitized degradation of 4-chlorophenol (4-CP) under visible light (lambda greater than or equal to 450 nm) irradiation in an aerated aqueous medium at pH 12 was studied using an immobilized photosensitizer, aluminum tetrasulfophthalocyanine, on a commercial resin Amberlite IRA 400. The catalyst exhibited strong adsorption toward 4-CP, but the adsorption led to an exponential decrease in both the initial rate and the apparent first-order rate constant, as measured by 4-CP loss in the bulk solution. Several intermediates were formed from 4-CP oxidation, including fumaric acid, benzoquinone, and hydroquinone, which were adsorbed strongly on the catalyst and lowered the photosensitized reaction. Addition of H2O2 was found to be an efficient way to eliminate the colored intermediates and consequently recover the catalyst activity. The immobilized sensitizer was stable and could be used repeatedly in the presence of H2O2. The optimal loading of the photosensitizer in the catalyst was about 1.0 wt %.