The electrosorption behavior of tetramethylthiourea (TMTU) on a mercury electrode from 1.0, 0.5, and 0.1 M NaClO4 solutions containing 0.1 M tert-butanol (TB) is described by means of adsorption isotherm constants. The adsorption parameters for the double layer were Calculated from a double-layer differential capacity measurement extrapolated to zero frequency. The Values of the relative surface excess increase along with the concentration of NaCiO(4). It seems that the adsorption of TMTU is determined by an interaction between adsorbed molecules. The presence of TB in the adsorption layer distinctly influences the parameters of the inner layer. (c) 2009 Elsevier Inc. All rights reserved.