The dynamic magnetic susceptibility response of magnetic nanoparticles was used to independently determine the viscosity of the dispersed and continuous phases of oil-in-water emulsions in situ. Cobalt ferrite nanoparticles coated with oleic acid (OA) or poly(ethylene glycol) (PEG) were prepared and mixed with emulsions, where they partitioned to the dispersed oil phase or continuous water phase, respectively. Emulsions with a range of dispersed-phase volume fractions were prepared and characterized using the nanoparticles and conventional rheometry. Conventional rheometry showed the expected increase in emulsion viscosity with increasing volume fraction of the dispersed oil phase. In contrast, the viscosity felt by the oleic acid coated nanoparticles partitioned to the dispersed oil phase was found to be independent of the volume fraction of the discontinuous phase and quantitatively similar to the bulk phase viscosity of mineral oil. Similarly, the viscosity felt by the PEG coated nanoparticles dispersed in the continuous water phase was also found to be independent of the volume fraction of the dispersed oil phase and quantitatively similar to the bulk viscosity of water. These results demonstrate how magnetic nanoparticles can be used to directly characterize the viscosity of the dispersed and continuous phases of emulsions in situ. (C) 2016 Elsevier Inc. All rights reserved.