In alkaline solution a strongly passivating layer is formed close to a copper metal surface during a positive sweep to potentials beyond approximately +0.5 V (vs. SCE). Simultaneous ultrasonic irradiation during the voltammetric scan promotes the anodic dissolution of copper and greatly increases anodic currents in the less-positive potential region 0-0.5 V (vs. SHE), which corresponds to the formation of a thicker, more porous oxide/hydroxide layer, but does not prevent this passivation process from occurring at potentials above 0.5 V in sodium hydroxide solution up to 3 mol dm(-3) concentration. The passivating layer requires maintenance of oxidising conditions, and so the removal of the protective oxide/hydroxide coating, which is effectively performed by ultrasound, facilitates reductive loss of the passivating layer sufficiently readily that an anodic current can be seen during the reduction scan between +0.5 and 0 V while scanning from the positive limit in a negative direction. In 1 M NaOH a curve-crossing phenomenon occurs in silent voltammetry in which scan reversal before the onset of passivation produces a higher anodic current in the reverse (reductive) direction than during the forward positive scan before reversal. Also at this alkali concentration, when the positive range is scanned into the passivation region the maximum anodic peak current in silent voltammetry is independent of scan rate over the range from 5 to 80 mV s(-1). Presonication of the copper surface prior to silent voltammetry greatly increases the peak currents and enhances peak definition in the copper voltammogram, suggesting general activation of the metal surface by ultrasound. The oxidation processes continue to occur at similar potentials, but there is a new peak in the reduction scan, with some shift of reduction potentials. (C) 2004 Elsevier B.V. All rights reserved.