A novel procedure has been developed for spectrophotometric determination of anionic surfactants in water using a solenoid micro-pump as fluid-propulsion device. The proposed method is based on substitution of methyl orange (MO) by anionic surfactants in the formation of an ion-pair with the cetyl pyridine ion (CPC⁺) at pH 5.0. The flow network comprised four solenoid micro-pumps which, under microcomputer control, enabled sample and reagent introduction, and homogenisation in the reaction zone. The system is flexible and simple to operate and control, and sensitive and precise. The analytical plot for the anionic surfactant was linear between 1.43×10⁻⁶ and 1.43×10⁻⁵  mol L⁻¹ (0.5 to 5.0 mg L⁻¹; R=0.997, n=5). The relative standard deviation was 0.8% (n=11) for a sample containing 5.74×10⁻⁶ mol L⁻¹ (2 mg L⁻¹) surfactant. The limit of detection was 9.76×10⁻⁸ mol L⁻¹ (0.034 mg L⁻¹) and the sampling throughput was 60 determinations per hour. The results obtained for washing-water samples were comparable with those obtained by use of the reference method, and no significant differences at the 95% confidence level were observed.