The structure of thick lead dioxide deposits (approximately 1mm) formed in conditions likely to be met at the positive electrode during the charge/discharge cycling of a soluble lead-acid flow battery is examined. Compact and well adherent layers are possible with current densities >100mAcm−2 in electrolytes containing 0.1–1.5M lead(II) and methanesulfonic acid concentrations in the range 0–2.4M; the solutions also contained 5mM hexadecyltrimethylammonium cation, C16H33(CH3)3N+. From the viewpoint of the layer properties, the limitation is stress within the deposit leading to cracking and lifting away from the substrate; the stress appears highest at high acid concentration and high current density. There are, however, other factors limiting the maximum current density for lead dioxide deposition, namely oxygen evolution and the overpotential associated with the deposition of lead dioxide. A strategy for operating the soluble lead-acid flow battery is proposed.