The reduction of NO by CO in presence of O-2, a reaction of great technological importance, was investigated on porous polycrystalline Rh catalyst-electrodes deposited on YSZ (Y2O3-stabilized-ZrO2) an O2- conductor. It was found that application of current or potential between the Rh catalyst electrode and a Au counter-electrode enhances the rate of NO reduction and CO2 formation by up to a factor of 20. These rate increases are strongly non-Faradaic with apparent Faradaic efficiencies, Lambda, up to 20, manifesting the effect of Electrochemical Promotion or Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA). The Rh catalyst electrodes were subsequently promoted in a classical way, via dry impregnation with NaOH, followed by drying and calcination. The thus Na-promoted Ph films were found, as expected, to exhibit much higher catalytic activity than the unpromoted films, with a pronounced decrease in their light-off temperature from 440 to 320 degreesC. The effect of Electrochemical Promotion was then studied on these, already Na-promoted Ph catalyst. Positive (+1 V) potentials were found to further increase the rate of NO reduction by up to a factor of 4 with a Faradaic efficiency up to 20 and concomitant reduction in light-off temperature down to 260 degreesC. This is the first demonstration of electrochemical promotion on an already promoted catalyst surface. (C) 2000 Elsevier Science Ltd. All rights reserved.