The voltammetry of various well-defined systems in acetonitrile solution has been studied using both micro and macroelectrodes in the presence of power ultrasound. A simple model is established which quantifies the mass transport observed under these conditions; this assumes that the effect of ultrasound is to promote mixing within the bulk of the solution up to within a certain distance of the electrode surface. Thus the ultrasound serves to thin the diffusion layer which would exist at the corresponding electrode under silent conditions. The relative enhancement of transport limited currents by ultrasound is dependent on the size of the electrode; for micrometre-sized electrodes the steady state limiting current observed tends to that predicted under silent conditions whereas for large electrodes a thin, steady-state diffusion layer is seen with a thickness which is power dependent. In addition to steady-state experiments, a.c. impedance measurements and potentials steps are used to verify the model proposed.