A liquid chromatographic method using gradient elution and diode array UV detection for the trace level determination of seven synthetic pyrethroids is described. The procedure involves automated on-line preconcentration on precolumns containing octadecylbonded silica using a Prospekt. Although the analytes display a "mixed" hydrophobic/hydrophilic behaviour, both breakthrough on the precolumn and adsorption to inner walls and surfaces can be prevented by adding Brij-35, a neutral surfactant, to the aqueous sample. The procedure has been tested for the analysis of surface water; detection limits typically are at the sub-μg/l level and repeatability is excellent. The automated system is robust and there are no maintenance problems. © 1994 Springer-Verlag.