Abstract Nickelic hydroxide is an important metallurgical reagent used for precipitating cobalt from nickel sulfate solutions. Existing methods of preparation of nickelic hydroxide involve electrolytic oxidation of Ni(OH)2 or the use of strong oxidizing agents such as chlorine, ozone and persulfate. The present paper describes a new method for chemical preparation of nickelic hydroxide, developed in INCO laboratories, by a technique consisting of reacting nickelous hydroxide suspensions with a slurry of calcium sulfite in the presence of air or oxygen. The main advantage of the process is the cost of reagents and an absence of soluble by-products (e.g. Cl−). The chemistry of this process is unusual since the redox potential obtained during the reaction exceeds the theoretical potential of the oxygen electrode. The effect of CaSO3, therefore, is not just a catalytic acceleration of the reaction Ni(OH)2 + O2, for the SO32− must be involved in the stoichiometry of the overall reaction by producing, in the initial phase, some strongly oxidizing intermediate species, most likely the peroxomonosulfate ion SO5 2 −.