During electrowinning of cobalt in a chloride electrolyte, in addition to anodic chlorine and oxygen evolutions, precipitation of a black substance was observed on the dimensionally stable anode. The black substance was investigated by means of X-ray diffraction and found to be trivalent cobalt oxy-hydroxide (alpha-CoOOH). The deposition of cobalt oxy-hydroxide and possible ways to prevent this deposition were studied. Experiments showed that the deposited cobalt oxy-hydroxide could be dissolved cathodically. The amount of charge for cathodic dissolution was used to estimate the deposited amount of cobalt oxy-hydroxide. The partial current density for the cobalt oxy-hydroxide formation increased with increasing anodic potential, indicating an electrochemical deposition reaction [Co2+(aq) + 2H(2)O -> 3H(+)(aq) + CoOOH + e -]. In agreement with thermodynamics, the deposition of cobalt oxy-hydroxide decreased with decreasing pH. The deposition was no longer observed at low pH (pH = 1.0), in good agreement with the thermodynamic calculations. The results showed that the cathodic pulses on the anode during electrowinning can be used to prevent accumulation of cobalt oxy-hydroxide on the anode. It was also found that the addition of small amounts of hydrogen peroxide suppressed the deposition of cobalt oxy-hydroxide. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3515076] All rights reserved.