A numerical model is used to examine how the uniformity of current distribution at a flat electrode can be improved using a coplanar auxiliary electrode. A boundary-element code, based on potential theory and Tafel kinetics, determines the current that must be passed at a surrounding auxiliary electrode to minimize nonuniformity on a round cathode. Nonuniformity is quantified by the root-mean-square deviation from the mean current density over the cathode surface. The importance of geometric factors such as the size and position of the auxiliary electrode is examined. Applications to plating in the electronics industry and to rotating-disk electrodes are discussed. © 1990, The Electrochemical Society, Inc. All rights reserved.