Time-differentiation of the frequency response observed with electrochemical quartz crystal microbalance (EQCM) was applied for the analysis of the electrode processes for the Au electro-dissolution and deposition in acidic aqueous solution containing free chloride ion (Cl-). Evaluation for the values of the apparent equivalent molar mass per second proved the generation of the Au(I) species during the oxidation and reduction reactions of Au electrode in the presence of Cl- at a level of 10 mM, which has not been reported so far. It was found that the electron-transfer reactions composed of Au(0). Au(I) and Au(III) species are participated in the overall Au electro-dissolution and deposition reactions in the applied potential range from -0.10 to 1.50 V (vs. Ag/AgCl). (C) 2011 Elsevier B.V. All rights reserved.