Nickel oxide is prepared by heating pure nickel powder with oxygen at a temperature above 400°C. In some commercial processes, green nickel oxide is made by heating a mixture of nickel powder and water in air at 1,000°C. Adding some nickel oxide to the above mixture enhances the rate of reaction. An alternative method of preparation of the green oxide involves thermal decomposition of an oxo acid salt of nickel at elevated temperatures. Thus, nickel nitrate, nickel sulfate or, more conveniently, nickel carbonate when heated at 1,000°C, yields the green oxide. The black oxide, on the other hand, is produced at a lower temperature from incomplete calcination of the carbonate or nitrate salt at 600°C. The oxygen content of the black form is slight-ly greater than its green counterpart.