In industrial manufacture of chlorate, current efficiency at the steel cathode is affected by different surface deposits. Comparative laboratory and field investigations show that presence of calcium, magnesium, carbonate and sulfate ions in the electrolyte influence structure and initial growth of the basic calcic layer. Surface analysis was performed using scanning electron microscopy and microanalysis (SEM/EDS) with complementary X-ray diffraction analysis. The calcium ion concentration influences the structure and crystallinity of the deposit even at low levels, 10-70 ppm (w). Higher concentrations yield denser deposits and more distinct crystals. Additions of magnesium ions reduce the average particle size and increase the insulating properties. The presence of carbonate in the electrolyte increases the amount of magnesium in the deposit, whereas sulfate has the opposite effect. A combination of sulfate and carbonate reduces deposition of both calcium and magnesium containing species on the cathode. The surface layer is made up of a complex mixture of phases. Layer thickness and crystallinity increase with time. Sodium chloride and sodium chlorate were identified in the deposit. Diffraction data indicate the formation of hydroxide phases and a hydrated calcium chlorate. No evidence supports the presence of any simple calcium/magnesium hydroxide, chloride or sulfate in the deposit.