The effect of potassium hydrogen phthalate (C8H5KO4) as a special additive on the one-step electrodeposition of single-phase CuInS2 thin films from acidic solution (pH 2.5) was investigated in detail. The XRD. SEM and UV-vis-NIR characterization confirms that the addition of an adequate concentration of C8H5KO4 (23 mM) to the electrolytic bath containing 12.5 mM Cu2+, 10 mM In3+. 40 mM S2O32- and 100 mM LICI can contribute greatly to the controllable growth of pure chalcopyrite CuInS2 films with uniform surfaces and an ideal band gap of approximately 1 54 eV Complexation studies of C8H5KO4 with Cu2+ and In3+ in electrolytic solutions indicated that C8H5KO4 can complex Cu2+ more strongly than In3+ and move the electrode potentials of Cu2+ and In3+ near each other as determined by polarization analysis. Furthermore, the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) analysis performed in a series of solution systems revealed a three-step reaction mechanism for CuInS2 deposition and considerable adsorption of C8H5O4- and Cu(C8H5O4)(+) to the cathode surface. This deposition shows that the synergetic effects of complexation and adsorption originated from the additive on the Cu2+ electro-reduction, thus promoting the co-deposition of copper, indium and sulfur in the form of single-phase CuInS2. (C) 2010 Elsevier Ltd. All rights reserved.