Systematic studies have been made to understand the oxidation behaviour and dissolution reaction mechanisms operative while pure copper, zinc and lead sulphide minerals and mixtures, and single concentrates/mixtures are treated by oxidative ammonia leaching. The information thus obtained is useful for characterisation of both the reactants and the products. During the leaching of bulk concentrate, it is possible to obtain high extraction rates ( > 95%)for copper from chalcopyrite, zinc from sphalerite, and lead from galena. Pyrite remains in the leach residue along with oxidised lead compounds and goethite is formed due to oxidation of iron from chalcopyrite.