The adsorption of thiourea on polycrystalline and monocrystalline copper electrodes of basal indices: (111), (100) and (110) was studied by radiometric and electrochemical methods. The measurements were carried out in perchloric acid solution at ambient temperature. Thiourea labeled with C-14 was used in the experiments. The dependence of the surface concentration of thiourea on the electrode potential and on the bulk concentration was determined for each studied surface. Based upon the obtained radiometric data it can be stated that adsorption of thiourea on investigated copper electrodes takes place in the entire range of the applied potential range, The adsorption process is mainly reversible at constant potential which allowed us to determine adsorption isotherms and also to calculate values of the Gibbs energy of adsorption for the copper surfaces studied. The calculated values of surface concentration of the adsorbed species were different and dependent on a copper plane whereas the Gibbs energy of adsorption was practically the same. This means that the energy of interaction of TU molecules with adsorption sites on copper electrodes is similar and the differences in surface concentrations Gamma (max), are mainly due to the different number of adsorption sites, i.e. the surface atom density of individual planes of Cu electrodes.