Oxygen-transfer enhancement has been observed in the presence of colloidal dispersions of magnetite (Fe3O4) nanoparticles coated with oleic acid and a polymerizable surfactant. These fluids improve gas?liquid oxygen mass transfer up to 6-fold (600%) at nanoparticle volume fractions below 1% in an agitated, sparged reactor and show remarkable stability in high-ionic strength media over a wide pH range. Through a combination of experiments using physical and chemical methods to characterize mass transfer, it is shown that (i) both the mass transfer coefficient (kL) and the gas?liquid interfacial area (a) are enhanced in the presence of nanoparticles, the latter accounting for a large fraction of the total enhancement (80% or more), (ii) the enhancement in kL measured by physical and chemical methods is similar and ranges from 20 to 60% approximately, (iii) the enhancement in kL levels off at a nanoparticle volume fraction of approximately 1% v/v, and (iv) the enhancement in kLa shows a strong temperature dependence. These results are relevant to a wide range of processes limited by the mass transfer of a solute between a gas phase and a liquid phase, such as fermentation, waste treatment, and hydrogenation reactions.