Extensive cycling of the soluble lead flow battery has revealed unexpected problems with the reduction of lead dioxide at the positive electrode during discharge. This has led to a more detailed study of the PbO2/Pb2+ couple in methanesulfonic acid. The variation of the phase composition (XRD) and deposit structure (SEM) have been defined as a function of current density, Pb2+ and H+ concentrations, deposition charge and temperature as well as the consequences of charge cycling. Pure alpha-PbO2, pure beta-PbO2 and their mixtures can be deposited from methanesulfonic acid media. The alpha-phase deposits as a more compact, smoother layer, which is well suited to charge cycling. While the anodic deposition of thick layers of PbO2 is straightforward, their reduction is not; the complexities are explained by an increase in pH within the pores of the deposit. The results suggest that operating the battery at lead(II) concentrations <0.3 M and elevated temperatures should be avoided. (C) 2009 Elsevier Ltd. All rights reserved.