MION-46L is an ultrasmall monocrystalline superparamagnetic (SPM) iron oxide that is of current interest as an MR contrast agent. It is believed to consist primarily of small maghemite or magnetite crystals that possess a SPM moment, but the exact magnetic properties and related mechanisms of T(1) and T(2) proton relaxation enhancement are not well understood. We have obtained a comprehensive data set consisting of magnetization curves, EPR spectra, and 1/T(1) and 1/T(2) nuclear magnetic relaxation dispersion (NMRD) profiles for this contrast agent. The magnetization curves show a primary curvature consistent with a SPM moment of 10,300 Bohr magnetons (BM) per particle. In addition, there is a secondary high-field curvature that is consistent with a smaller SPM moment of 1600 BM, which may be responsible for the observed high-field increase in 1/T(2). Finally, there appear to be a considerable number of paramagnetic ions present that are needed to account for the high-field increase in magnetization, and that can provide an alternative explanation for the magnitude of the low-field T(1) plateau. This "three-phase model" appears to be successful in explaining in a self-consistent and quantitative manner the combined results of the magnetometry, relaxometry, and EPR studies. Magn Reson Med 42:379-384, 1999. Published 1999 Wiley-Liss, Inc.