A detailed experimental study is described which was conducted to determine the rate of oxygen gas-liquid mass transfer within the packed bed of heap bioleach reactors at different temperatures (22-68 degrees C), using the Na(2)SO(3) method. The raw data was analysed using a simplified film mass transfer model, making corrections for oxygen solubility in concentrated solution and for increased water vapour partial pressure at elevated temperatures. The results compared favourably against two independent experimental leach studies, indicating k(L)a values between 33 and 46 h(-1). The value varied with the particle size distribution of the packing, with k(L)a assuming larger values for those packings that had a higher fines contents. While k(L)a increases with temperature, the solubility of oxygen decreases simultaneously, resulting in net mass transfer rates being relatively unaffected by temperature in the range studied. This indicates that thermophile heap bioleach reactors are likely to operate under gas-liquid mass transfer limitations, especially at high altitude. (C) 2010 Elsevier Ltd. All rights reserved.