An experimental study was conducted for assessing the formation of hydrocarbon gaseous compounds during bioleaching of copper sulphide ores with mesophilic microorganisms. Three different mineral samples were used: a pyrite concentrate, a chalcopyrite concentrate and a copper sulphide ore rock from El Teniente mine, Chile, containing 1.2% copper. Mineral samples were bioleached in 250 ml shake flasks containing 100 ml of basal medium inoculated either with a pure strain of Acidithiobacillus ferrooxidans or a natural bacteria consortium obtained from the acid leaching of the El Teniente copper ore in columns. Each sealed shake flask system was fed with a flow of synthetic air and the exit stream was passed through a column containing an adsorbent material and next through a water trap to avoid back-contamination with air from the environment. Compounds present in the adsorbent material after 90 days of bioleaching were analysed using a gas chromatography mass spectrometry technique with a procedure that detects 162 different hydrocarbon gaseous compounds. Experimental results permitted to identify the formation of significant amounts of five different types of hydrocarbon gaseous compounds: di-,tri- and penta-methyl benzene, 11th-ethyl alkane and branched alkanes. The number of formed gaseous hydrocarbon compounds increased in experiments conducted with the natural bacterial consortium, where their formation appears to be enhanced by the presence of heterotrophic microorganisms. This work provides the first experimental evidence of the formation of hydrocarbon gaseous compounds in bioleaching. (C) 2008 Elsevier B.V. All rights reserved.