A simple and rapid preconcentration technique for the determination of trace elements in waters is proposed. The technique is based on collecting trace elements on a membrane filter as complexes and dissolving the filter into organic solvents. An application of the technique to the spectrophotometric determination of phosphorus is described in this paper. Phosphorus was collected on a nitrocellulose or an acetylcellulose membrane filter as phosphomolybdenum blue in the presence of cationic surfactant, dodecyltrimethylammonium bromide, and the filter was dissolved in a small volume of dimethylsulfoxide(DMSO). By measuring the absorbance of the DMSO solutions at 710 nm vs. reagent blank, phosphorus ranging from 2.6 to 73 μg/l in water was satisfactorily determined. Silicate ion, an anionic surfactant {sodium di-(2-ethylhexyl) sulfosuccinate}, a non-ionic surfactant (Triton X-100), and highly concentrations of sodium chloride up to 0.5 M, do not interfere with this method.