Adsorption of N,N'-dimethylthiourea (DMTU) on mercury electrode from 0.1, 1 and 5M NaClO4 was studied as the function of electrode charge density and adsorbate bulk concentration. In the study, the experimental data obtained from the measurements of differential capacity of double layer were used, the measurements of zero charge potential and surface tension at the zero charge potential. In each system studied the values of the relative surface excess increase with an increase of the concentration of N,N'-dimethylthiourea and NaClO4. The adsorption parameters were obtained from the Frumkin, virial and modified Flory-Huggins isotherms. It was found that the values of free adsorption energy, interactions constants and integral capacity depends on the supporting electrolyte concentration. The strength of the surface bond formed between N,M-dimethylthiourea and the electrode surface and the influence of water present on the electrode surface in the obtained results of calculations were discussed. (c) 2006 Elsevier Ltd. All rights reserved.