A novel high-pressure sonoelectrochemical cell has been developed in order to study the effect of pressure on cavitation and acoustic streaming in electrochemical processes. The reversible one electron reduction of a solution of Ru(NH3)(6)(3+) in aqueous 0.1 M KCl at a 25 mu m diameter Pt microdisk electrode was studied under up to 60 bar pressure of argon and carbon dioxide and in the presence of ultrasound. The resulting cathodic current response was interpreted to be composed of a steady macroscopic streaming induced component and a transient spikelike component detected after the onset of the cavitation. The threshold for the cavitation process was strongly dependent on pressure and increased monotonically in the presence of argon. However, the threshold appeared at lower ultrasound power when pressurizing with carbon dioxide and even decreased at higher pressures (>40 bar) in the presence of CO2. The analysis of the observed phenomena is possible in terms of the mechanical pressure, the surface tension, and the formation of a liquid CO2 phase.