The Hg|HBr (aq) system has been studied in relation to current efficiencies of anodic mercurous bromide production, and to the photomicrographic examination of anodic deposits. Considerable differences in behaviour are observed from that of the Hg|HCl (aq) system, which are at least partly attributable to the increased incidence of disproportionation and critical effects of mercury(II) complexes in solution on the mode of crystal growth of the lowervalent halide. Observations are recorded of the anodic formation of mercurous iodide, sulphate, and carbonate.