Self-assembled monolayers of dodecanethiol (C12SH-SAMs) on polycrystalline gold were prepared under ultrasonic irradiation at 100W (the actual ultrasonic power intensity is about 0.1 W cm(-2) including the heat loss) for different time and investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV experiments show that the differential capacitance C-d values of the C12SH-SAM prepared under ultrasonic irradiation at 100 W (0.1 W cm(-2))for 15 min are independent of the scan rate, the thickness d value of this monolayer is 17.5 A, the tilt angle phi value of the molecules in this monolayer from the gold surface normal was calculated to he 30 degrees and the difference value of the Current density at -0.2 and 0.5 V (Delta i(p)) is only 0.69 mu A cm(-2). Front the EIS experiments, we find that the phase angle value at 1 Hz Phi(1Hz), Of the C12SH-SAM prepared under ultrasonic irradiation at 100 W (0.1 W cm(-2)) for 15 min is 89 degrees, the charge transfer resistance R-ct, value of this monolayer is 1.40 x 10(6) Omega cm(2) and the surface coverage theta value of this monolayer was calculated to be 99.997% from R-ct. These results indicate that the C12SH-SAM of almost defect-free structure and very low ionic permeability can be formed under ultrasonic it-radiation at 100 W (0.1 W cm(-2))in a short time (15 min). (c) 2007 Elsevier Ltd. All rights reserved.