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Cloud point extraction has been used for the preconcentration of uranium, prior to its determination by flow injection. The non-ionic surfactant employed was Triton X-I 14 and the reagent chosen to form a hydrophobic chelate of uranium was 1-(2-pyridylazo)-2-naphthol. The optimum conditions for the preconcentration and determination of uranium have been studied. This methodology has been applied to the determination of trace amounts of uranium in tap and river waters from Salamanca.Ordered media have been used in virtually all fields of analytical chemistry in order to improve existing methods and to develop new analytical procedures.'-6 During the last decade many separation processes mediated by organized media have been developed;7-9 in particular, applications including high-performance liquid chromatography (HPLC), extraction, gel filtration, ultracentrifugation and electrokinetic capillary chromatography have opened new possibilities for the separation of molecules in certain research areas such as analytical biotechnology, public health or the study of environmental pollutants.The use of micellar systcms in separation processes also allows the direct injection of untreated biological fluids (plasma, urine or saliva) or waste water samples in any hydrodynamic analytical system [ e . g . , HPLC or flow injection (FI)] which, in turn, will facilitate sample preparation and reduce the time of analysis.Aqueous micellar solutions of non-ionic surfactants such as alkylsulfinyl alcohols, alkylmethylsulfonediimines, dimethylalkylphosphine oxides and alkyl (or aryl) polyoxyethylene ethers exhibit a phase separation when the temperature is raised above a value which depends on the amphiphile nature and concentration. This is known as the cloud point phenomenon, and is not yet well understood. The studies point at different interpretations, from an increase in the micellar aggregation number with temperature to variation in the micellar interactions.10-14 When an aqueous solution of surfactant is heated above a critical temperature the solution suddenly becomes turbid (cloud point) and after a time interval (centrifugation can speed up this step) the solution separates into two transparent liquid phases: a surfactant-rich phase and an aqueous solution phase, which is in equilibrium with a surfactant concentration of close to the critical micellization concentration.The small volume of the surfactant-rich phase makes it possible to obtain high preconcentration factors (with recoveries ranging from 70 to 100%) for a wide variety of analytes, which can be solubilized or associated with the micelles. This surfactant-rich phase can be used for the preconcentration of some analytes prior to liquid chromatographic analysis with the obvious benefits (safety, cost, compatibility with micellar hydro-organic mobile phases, etc.). Thermolabile molecules, such as proteins or vitamins, can be included in the separation schemes, given the low cloud point of some non-ionic surfactants.The cloud point methodology has been used in diff...