The electrochemical investigations of 3 merkaptopropionate (3-MPA) as representative of organosulfur compounds and inorganic sulphide in pure seawater electrolyte and in natural water of Rogoznica lake. Two voltammetric techniques were used: square wave voltammetry and cyclic voltammetry. The interaction between mercury and sulphur compounds is the basis for their determination. The sulphide on the mercury electrode creates a layer of HgS which is insoluble and is reduced at a potential of -0.68 V, while the 3-MPA on the mercury electrode does not precipitate as HgS but rather creates a complex with Hg-3-MPA which is accumulated at potentials more positive than -0.4 V, and is reduced on a potential of about -0.44 V to -0.65 V regardin the concentration and pH. This different 3-MPA behaviour in relation to the sulphide allows the determination of 3-MPA simultaneously with sulphide and other organosulfur compounds during scan. Investigations have found that modification of parameters, such as accumulation time and deposition potential, can distinguish compounds similar to 3- MPA as well as in the sample of 3-MPA and sulfide mixtures. By changing the pH, the volatility of the sulfur compounds is changed, and the conditions in which the sulphide evaporates but the 3-MPA remains in the system. By changing the deposition potential, the type of compound that corresponds to 3- MPA of the type of compound similar as HgS can be distinguished. The 3-MPA and sulfide calibration curves and. the inclination of these curves, indicate a higher sensitivity of the sulfide than the organosulfur compound 3-MPA to the Hg electrode. From the calibration , the concentration of the organosumporic compounds in the natural sample of the Rogoznica lake taken from a depth of 2 and 7 m was determined. The surface layer confirmed the presence of 3-MPA organosumporic compounds, while in the anoxic layer was confirmed the presence of compounds that depleted HgS on the surface Hg electrodes.