An electrochemical quartz crystal microbalance is used for a continuous monitoring of the growth of the polymer poly (3,4-ethylenedioxy) thiophene tetrabutylammonium perchlorate (PEDOT-TBAP), electropolymerized in acetonitrile on a gold electrode of a 10 MHz AT-cut quartz crystal resonator. The surface acoustic impedance of the resonator is analyzed starting from the electrical admittance continuously measured by means of a network analyzer. Changes in the acoustic impedance suggest that a mechanical resonance phenomenon occurs during the electrodeposition. To determine the origin of this effect, the evolution of the physical properties of the coating is analyzed. This analysis shows a significant change of the viscoelastic properties of the coating during the electropolymerization and especially during the time interval of the suspected mechanical resonance. The effect of the surface roughness on the mechanical impedance of the coating is analyzed. This study seems to indicate that the changes in the effective viscoelastic properties of the coating are due to the changes in the surface roughness. The mechanical resonance effect also seems to be more a result of this change in the coating effective viscoelastic properties than of the growth of the coating thickness where coating viscoelastic properties are maintained constant. (c) 2006 The Electrochemical Society.