Abstract: In an acetate buffer medium of pH 5.4,a coordination complex of Al-CSPS with its mole ratio of 1 to 1 was formed by the reaction of Al(Ⅲ) with chlorosulfophenol S (CSPS),showing its absorption maximum at the wavelength of 649 nm (λ649) and absorption minimum at 552 nm (λ552).Values of absorbances at 649 nm and 552 nm of both the color solutions of Al(Ⅲ) and blank solutions were taken,and values of absorbance ratios and their differences (ΔAr) were calculated by the formula given.The measurement of absorbance ratio difference was used in the spectrophotometric determination of traces of aluminum in surface water.Linear relationship between values of ΔAr and mass concentration of Al(Ⅲ) was obtained in the range within 0.050 mg·L-1,with detection limit (3s/k) of 0.8 μg·L-1.Samples of surface water taken from Yangtze River,Huangpu River,Taihu lake and tap water,were analyzed by the present method,giving results in consistency with those found by GF-AAS.Values of recovery found by standard addition method were ranged from 93.0% to 107.0%.