Adsorption of 1,1,3,3-tetramethyl-2-thiourea(TMTU) on a mercury electrode from 1,0.5, and 0.1 M NaClO4 is described by means of the adsorption isotherms constants calculated from the surface pressure as a function of electrode charge density and adsorbate bulk concentration. The adsorption parameters for the double-layer were calculated based on the data from the differential capacity-potential curves. The values of the relative surface excess increase with an increase in the concentration of NaClO4, In each system studied, significantly smaller values of Gamma(S) for sigma(M) < 0 were obtained, compared with the corresponding values for sigma(M) greater than or equal to 0. At positive values of sigma(M), the values of DeltaGdegrees obtained from either the Frumkin or the Flory-Huggins isotherm are similar. However, the values of the parameter A obtained from the Flory-Huggins isotherm are lower. The dependence of phi(M-2) on the relative surface excess at a constant charge density is nonlinear. The rectilinear segments of these dependences obtained for smaller values of Gamma' were analyzed in order to calculate the electrostatic parameters of the inner layer. (C) 2004 Elsevier Ltd. All rights reserved.