An on-line preconcentration method, based on complex formation with ammonium diethyldithiphosphate (DDTP-NH4) and sorption on a C18-bonded silica reversed phase sorbent, for the determination of Pb, Cd and Cu in potable water and biological samples was investigated. Preconcentration factors of 25; 38 and 86 and detection limits of 7, 3" acceptdate="and 7 μg/L for Pb, Cd and Cu, respectively, were obtained, with sample throughputs of 26, 38" acceptdate="and 29 samples/h. The system was efficient at low pH ranges and good agreement between found and certified values was observed in the analysis of certified reference materials.