Activation enthalpies have been calculated for the first (DeltaH(1)(#)) and second steps (DeltaH(2)(#)) of the Zn(II)/Zn(Hg) electrode reaction in the absence and presence of thiourea (TU) in 1 mol l(-1) NaClO4 from the Arrhenius plots. The reduction of Zn(II) has been studied by means of the faradaic impedance method. The values of the real activation energy of the first step in the process of Zn(II) electroreduction in the presence of TU were decreased. (C) 2003 Elsevier B.V. All rights reserved.