AbstractA method employing computerized potentiometric stripping analysis has been developed in order to determine trace levels of thallium in various environmental samples. The method is based on the preconcentration of T1 onto a mercury film electrode (formed in situ) with a subsequent reoxidation of amalgamated T1. A deaeration of the solution as well as the use of EDTA have resulted in a significant improvement of the sensitivity. In a supporting electrolyte containing 0.02–0.07 M HNO3+ 0.02 M CH3COONa + 0.001 M EDTA + 6 × 10−5M HgCl2, the analytical signal versus concentration dependence is linear from 9 × 10−11to 2.5 × 10−8M T1 (preconcentration time from 5 to 30 min). A detection limit of about 5 × 10−11M T1 (0.01 μg/L) is achieved when using preconcentration for 30 min. Despite a relatively high acidity of the supporting electrolyte used (pH 1.0–2.5), EDTA is capable to suppress the interferences from PbIIions up to a 500‐fold excess in concentration and from CdIIand CuIIup to their 100‐fold excess, respectively. Nitric acid has been incorporated into the supporting electrolyte as its constituent in order to determine T1 directly in the samples decomposed by means of high pressure ashing. The applicability of the method is demonstrated on various samples such as tap water (spiked), rain water, river sediment or sewage sludge. The results agree well with those obtained by isotope dilution‐mass spectrometry. The method has also been tested to determine T1 in digested biological samples (e.g., pine needles, poplar leav