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 α-PbO2, pure β-PbO2 and their mixtures can be deposited from methanesulfonic acid media. The α-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 <0.3M and elevated temperatures should be avoided.