In this study the affinity of nickel ions on laterite gangue as a function of pH was investigated as an alternative reason for nickel losses observed during the bioleaching of nickel laterite ores. In microbiological extraction of nickel from laterites, dissolved nickel concentration in the leaching medium at the end was observed to be lower than at the beginning. It has been suggested that this was associated with the uptake of metal by the biomass of the fungi. This study demonstrated that electrosorption of nickel does contribute to metal loss during bioleaching. The surface charge and the type of minerals present in the ore gangue were shown to have an important influence on the adsorption capacity. The analysis of the process suggest that control of pH during the bioleaching process is critical in optimising the leaching process. (C) 2001 Elsevier Science Ltd. All rights reserved.