Adsorption and surface reactions of thiourea on silverized silver electrodes have been studied in 0.1 M HClO4 and 0.1 M NaClO4. Quantitative radiometric data obtained using C-14-labeled thiourea indicate that adsorption of thiourea takes place in the entire range of potentials accessible to surface studies on silver electrodes in the acidic and neutral solutions. The values of the thiourea surface concentration are very high, close to the maximum monolayer packing densities calculated from the molecular size of thiourea (ca. 6 x 10(14) molecules cm(2)). Reorientation of adsorbed molecules from perpendicular to parallel to the surface is a possible reason for the decrease in Gamma at potentials located on the positive side of the adsorption maximum. As shown by the surface/bulk-exchange experiments, adsorption is reversible in the potential range preceding and directly following the adsorption maximum. In this range, the surface process can be quite accurately described by the Frumkin adsorption model. When corrected for water displacement, the calculated value of the Gibbs energy of adsorption points to strong adsorption of thiourea on silver (chemisorption). At the most positive potentials, surface processes become irreversible, ultimately leading to the formation of chemisorbed sulfur-containing species that act as the electrode poison. Depending on the solution pH, an addition of chloride to the cell has different effects on the adsorption of thiourea. As a result of the Cl--caused reorientation of the adsorbate, the surface concentration of thiourea increases in the neutral solution. The opposite effect occurs in the acidic solution, where surface blocking by adsorbed chloride plays a dominant role.