A general thermodynamics-related model of oxygen solubility, previously developed for water and aqueous inorganic solutions, has been applied to bleaching solutions. The model allows estimations of dissolved oxygen concentrations to be made for different combinations of temperature, oxygen partial pressure and inorganic solute concentrations likely to be encountered during oxygen bleaching of pulp. Combination of the model with oxygen diffusion kinetics allows optimum temperatures for transport-controlled oxygen delignification rates to be estimated.