The high tendency of 5-nitropiazselenol for self-accumulation on thin mercury film electrode was used innovatively for determination of Se(IV) in natural waters. 5-Nitropiazselenol was formed by reaction between Se(IV) and 4-nitro-1,2-phenylenediamine in acidic solution and self-accumulation process was carried out directly from reaction media. The adsorbed 5-nitropiazselenol was stripped in HCl solution by DP cathodic potential scan. All parameters influencing the measurement were optimized and evaluated. Detection limit of this method is 0.06 ng mL(-1). Interferences of various cations and anions were studied. The adsorption tendency of some other piazselenols made by some aromatic ortho-diamines was also investigated. The problems arising from applying potential during accumulation process in natural waters analysis were discussed. This method was applied for determination of Se(IV) in natural waters collected from some internationally registrated lagoons south of Caspian Sea. The obtained results were compared with the results of ICP-AES and DPCSV after electrochemical preconcentration.