Oxygen solubility in ammoniacal solutions of the type used for pressure leaching of sulphide concentrates has been modelled as a function of oxygen partial pressure, temperature and the conjoint presence of several inorganic solutes. This was achieved by combining oxygen solubility models for pure water and single solute solutions with published experimental data for single solutes of relevance to sulphide leaching. Differences between the oxygen solubility effects of dissolved molecular ammonia and ammonium salts were noted and analysed. Justification was made for assuming no distinction between effects of ammine cations and normal aqueous (solvated) cations on oxygen solubility. Applications of the model to industrial solutions are discussed, including estimation of oxygen concentrations and optimum process temperatures for mass transport of dissolved oxygen. (C) 2000 published by Elsevier Science Ltd. All rights reserved.