The oxidative leaching of a nickel sulphide concentrate in sulphuric acid solution was investigated. Both NaNO3 and Fe(III) were investigated as possible oxidizing agents. Nitrate was found to give better leaching rates than Fe(III). The reaction followed a surface controlled reaction mechanism, 1 -(1 - X)(1/3) = kt, during the initial stages of leaching. The activation energy of the chemical reaction was calculated as 88 kJ/mol in the temperature range 60 degrees C to 95 degrees C. Sulphur forms as a product during leaching and tends to inhibit the leaching rate, probably due to the formation of a sulphur layer on the concentrate particles during this stages of the reaction, a diffusion controlled mechanism became operative.