A leaching study of metallic cobalt in an acid medium has been carried out using a rotating disc geometry. The surface reaction involving hydrogen discharge and the diffusion of hydrogen ions through the boundary layer, together, were found to be responsible for the rate of dissolution. The apparent surface concentration of cobalt in solution is constant irrespective of the disc rotation speed. A typical value for c s, the cobalt ion surface concentration, at the bulk hydrogen concentration of 0.05 mole per litre, was found to be 8.32 × 10 −4 mole per litre. The cobalt surface concentration is only a very weak function of the bulk hydrogen ion concentration. The apparent activation energy for cobalt dissolution under the conditions of the experiments was found to be 4.0 kcal/mol.