The effect of thiourea (TU) concentration on copper electrodeposition was studied employing cyclic voltammetry and electrochemical crystal quartz microbalance techniques. It was found that the polarization effects are related to the type of species generated in the electrode interface. Due to formation of formamidine disulfide (FDS) at concentrations greater than 10(-4) mol dm(-3), the electrode surface is blocked, and copper electrodeposition shifts towards more negative potential values (polarizing effect). Lower concentrations favors the formation of Cu(I) -TU contributing to the de-polarization of copper electrodeposition reaction.