Polyelectrolytes are used in drinking water treatment and potentially remain as contaminants in the treated water. A method capable of quantifying these polymers at concentrations well below I mg/L is needed. Copolymerization of acrylamide or DADMAC with small amounts of an amine-functional monomer yields a flocculant that can be tagged with a reactive fluorophore before or after use and detected at low concentration. Tagging with fluorescein isothiocyanate yields polymers having high fluorescence intensity. The fluorescein chromophore is light-sensitive and suffers interference from organic matter in natural waters, but ways of overcoming these problems were developed. Size-exclusion chromatography separates residual polyelectrolyte from interfering impurities and allows it to be quantified by fluorescence. The detection limit for tagged polyDADMAC was estimated to be similar to10-40 mug/L in clean water and higher in treated potable water. Polymer residuals were consistently detected in water treated with polyDADMAC alone, but not in water treated with polyDADMAC or polyacrylamide together with alum. Use of polyDADMAC as a sole coagulant should therefore be strictly regulated. (C) 2004 Elsevier B.V. All rights reserved.