Abstract: In a phthalate buffer of pH 2.4 and in the presence of polyvinyl alcohol,a hydrophobic ionic association compound was formed between ethyl violet and sodium dodecyl benzene sulfate (SDBS),showing its absorption maximum at the wavelength of 550 nm.Based on this fact,a new on-line method for determination of an anionic surfactant,SDBS,by FI-spectrophotometry was proposed.Range of linearity for SDBS concentration was found between 0.08-3.50 mg·L-1.Detection limit (3S/N) found was 4.39 μg·L-1.Precision was tested with standard solution containing 2.4 mg·L-1 SDBS for 14 determinations,giving value of RSD of 1.72%.Four samples of environmental water were analyzed by the proposed method for their contents of SDBS,giving results in consistency with the results found by the GB standard method.Values of recovery found by standard addition method were in the range of 95.0%-107.4%.