The cathodic stripping voltammetry is used to determine arsenic in presence of copper and sodium diethyldithiocarbamate (DDTC-Na). The method showed several advantages in comparison to analogous one without DDTC-Na: higher sensitivity, substantially lower detection limit already at short deposition time, better repeatability and reproducibility. Detection limit of As(III) as low as 0.06 nM (0.004 ppb) possible to obtain with deposition time as short as 50 s. The lower detection limit is achievable at longer deposition time. The repeatability measured as relative standard deviation is 2%. Interferences from selected inorganic ions and surfactants were examined. Unique application of DDTC-Na was found as useful modification of the CSV method. The proposed method is successfully applied to arsenic determinations and speciation in drinking water samples. (c) 2006 Elsevier B.V. All rights reserved.