Formation of hydrogen peroxide (H2O2), an oxidising agent stronger than oxygen, by pyrite (FeS2), the most abundant metal sulphide on Earth, during grinding was investigated. It was found that pyrite generated H2O2 in pulp liquid during wet grinding and also the solids when placed in water immediately after dry grinding. Type of grinding medium on formation of hydrogen peroxide revealed that the mild steel produced more H2O2 than stainless steel grinding medium, where Fe2+ and/or Fe3+ ions played a key role in producing higher amounts of H2O2. The effect of grinding atmosphere of air and N-2 gas showed that nitrogen environment free from oxygen generated more H2O2 than air atmosphere suggesting that the oxygen in hydrogen peroxide is derived from water molecules. In addition, the solids after dry grinding producing more H2O2 than wet grinding indicate the role of pyrite surface or its catalytic activity in producing H2O2 from water. This study highlights the necessity of relooking into the electrochemical and/or galvanic interaction mechanisms between the grinding medium and pyrite in terms of its flotation behaviour. (C) 2013 Elsevier Ltd. All rights reserved.