The effect of electrochemical promotion of catalysis was investigated for the water-gas shift reaction over porous Pt catalyst electrodes interfaced with 8%mol Yttria-stabilized Zirconia. A fuel cell type electrochemical reactor was used at temperatures from 300 degrees C to 400 degrees C, under P(H2O)/P(CO) ratio values from 2.85 to 31. A negative order dependence of the catalytic reaction rate on P(CO) and a positive one on P(H2O) was found under open-circuit and polarization conditions. Positive potential application (+2.5 V), i.e., O(2-) supply to the catalyst surface, causes a small decrease in the catalytic reaction rate, while negative potential application (-1.5 V) results in a pronounced rate increase, up to 200%, with apparent faradaic efficiency values up to 110. The rate increase obtained with negative polarization can be attributed to the weakening of the Pt-CO bond strength but also, to the increase in surface concentration of oxygen ion vacancies near the Pt-gas-support three-phase boundaries necessary for water dissociation. (C) 2011 Elsevier Inc. All rights reserved.