A radiotracer technique combined with cyclic voltammetry has been used for the investigation of underpotential deposition of thallium onto (111), (110) and (100) single crystal silver electrodes from perchloric acid solutions (0.1 M). Two modes of thallium adsorption have been found: the weak adsorption without charge transfer and the strong adsorption with the charge transfer, at potentials less negative and more negative, respectively, than those at which the upd peaks occur. The electrosorption valency scans the range from zero to one. A dynamic equilibrium between the adsorption and the desorption processes exists at all potentials studied. Radiometrically determined monolayer surface concentration of thallium does not depend on the crystallographic orientation of silver electrodes. Apparently, a nonepitaxial thallium monolayer is formed, but it is less dense than the close packed structure and leaves some surface active sites on silver electrode free for hydrogen evolution.