Self-heating of sulphides poses safety, environmental and economic concerns to the mining and metallurgical industries, especially when sulphide ores or concentrates have to be stored or transported. The first target of this investigation is the self-heating of pyrrhotite (Po), the most commonly suspected mineral to trigger the phenomenon because of its fast oxidation in moist air. Following a standard experimental protocol, self-heating was assessed on samples of Po mixed with sand as an inert material. With increasing Po content, self-heating rates progressively increased but the samples visually appeared less and less oxidized. A high Po content, it was hypothesized, may give a more reducing environment favouring formation of hydrogen sulphide (H(2)S). The presence of this gas could be significant for self-heating for several reasons which are discussed. The hypothesis was tested by including copper pieces to detect the gas by forming copper sulphide, and adding copper sulphate in excess which suppressed self-heating, apparently by reacting with and removing the gas. (C) 2009 Elsevier Ltd. All rights reserved.