In situ microwave activation of electrochemical processes is possible by self-focusing of intense microwave radiation into a region close to the electrode\solution (electrolyte) interface of a microclectrode placed inside a microwave cavity. A systematic study of the microwave activation effects in electrochemical processes is reported for two redox systems, Fe(CN)(6)(3-/4-) and Ru(NH3)(6)(3+/2+), in aqueous KCl solution. Platinum microelectrodes of 100, 50, and 25 mum diameter are employed and at the 25 mum diameter electrode, extreme current enhancements of up to three orders of magnitude are detected. A typical Nernst diffusion layer thickness in aqueous solution of less than 100 nm can be achieved routinely and, consequently, high temperature steady state voltammetry is possible in the sub-millisecond time domain. Volatile reagents reduce the efficiency of this effect and therefore a steam bubble mechanism is proposed to explain the observations. Microwave effects on the rate of interfacial electron transfer are discussed. (C) 2004 Elsevier B.V. All rights reserved.