The quantitative extraction of cationic surfactant (CS⁺) in river sediments was studied. Further, the developed method was applied to the spectrophotometric determination of CS⁺ in urban river sediment samples by solid-phase extraction with membranes. A mixture of methanol and hydrochloric acid was proposed as an eluent. Dried sediment was digested in the eluent under ultrasonic irradiation. After elution, the eluent was evaporated to almost dryness. The residue was dissolved in a small volume of methanol and diluted to a certain volume with water. The pH of the solution was adjusted to 4 - 5 to separate iron and some other metals as precipitates of hydroxides. The solution was passed through two-piled membranes: first glass-fiber and then polytetrafluoroethylene (PTFE) membranes. A small volume of methanol was passed through the membranes to elute any CS⁺ retaining on the membranes. After passing the methanol solution through a cationic exchange resin column, the retained CS⁺ was eluted with methanol containing a high concentration of sodium chloride. Water, Bromophenol Blue (BPB) and hydrochloric acid were added to the solution. The solution was passed through a mixed cellulose ester membrane filter to retain an ion associate of CS⁺·BPB^-. The retained ion associate was dissolved in a small volume of N,N-dimethylformamide together with the membrane filter, followed by the addition of triethanolamine to make the solution alkaline. The absorbance due to BPB^2- was measured at 603 nm against a reagent blank. This method was applied to the determination of CS⁺ in river water and sediment. A cationic surfactant in sediments at 10^-5 mol kg^-1 levels was detected with satisfactory precision. It was found that CS⁺ was about 500-fold enriched in the sediment from water at the place where domestic wastewater was discharged.