During development of an undivided flow battery based on the Pb(II)/Pb and PbO2/Pb(II) couples in aqueous methanesulfonic acid, it was noted that battery performance might be improved by additives that (i) decrease the roughness of the lead deposit at the negative electrode and (ii) enhance the kinetics of the Pb(II)/PbO2 couple at the positive electrode. This paper reports the study of sodium ligninsulfonate and polyethylene glycol as potential levelling agents for lead and of three inorganic ions as possible catalysts for the Pb(II)/PbO2 couple. The addition of I g dm(-3) ligninsulfonate leads to uniform deposits without the tendency to form dendrites but leads to a slight decrease in both charge and energy efficiency for the battery. Only nickel(II) reduced the overpotential for PbO2 deposition but again it has an adverse influence on the energy efficiency. (c) 2005 Elsevier B.V. All rights reserved.