Small-angle neutron scattering and dynamic and static light scattering measurements were used to probe the structures of aqueous and organic-solvent-based magnetic fluids comprising dispersed magnetite nanoparticles (similar to 10 nm in diameter) stabilized against flocculation by adsorbed alkanoic acid layers. A core-shell model fitted to a set of neutron scattering spectra obtained from contrast variation experiments allowed the determination of the iron oxide core size and size distribution, the thicknesses of the surfactant shells, and the spatial arrangement of the individual particles. The magnetic colloidal particles appear to form compact fractal clusters with a fractal dimension of 2.52 and a correlation length of similar to 350 Angstrom in aqueous magnetic fluids, consistent with the structures of clusters observed directly using cryo-TEM (transmission electron microscopy), whereas chainlike clusters with a fractal dimension of 1.22 and a correlation length of similar to 400 Angstrom were found for organic-solvent-based magnetic fluids. The differences in cluster structure were attributed to the relative strengths of the particle-particle interaction energies. Weak interactions in the organic-solvent based systems dictate the formation of small structures for which the apparent fractal dimensions are naturally small, whereas significantly stronger interparticle interactions in aqueous magnetic fluids result in larger, more compact clusters with higher fractal dimensions. The growth of the aqueous clusters beyond a certain size was inhibited by an increasingly high energy barrier (balance between repulsive electrostatic and attractive van der Waals interactions) with increasing cluster size. The aqueous clusters were stable against further growth when diluted with a surfactant solution but grew in time when diluted with pure water. In the latter case, the loss of part of the stabilizing secondary surfactant layer to the aqueous phase to satisfy thermodynamic partitioning constraints led to a destabilization in its dispersion. Light scattering studies indicated a change in the fractal dimension from 2.52 to about 1.20 as the clusters grew.