UNLABELLED: Superparamagnetic iron oxide nanoparticles (SPIONS) were synthesized by thermal decomposition of an organometallic precursor at high temperature and coated with a bi-layer composed of oleic acid and methoxy-polyethylene glycol-phospholipid. The formulations were named SPION-PEG350 and SPION-PEG2000. Transmission electron microscopy, X-ray diffraction and magnetic measurements show that the SPIONs are near-spherical, well-crystalline, and have high saturation magnetization and susceptibility. FTIR spectroscopy identifies the presence of oleic acid and of the conjugates mPEG for each sample. In vitro biocompatibility of SPIONS was investigated using three cell lines; up to 100μg/ml SPION-PEG350 showed non-toxicity, while SPION-PEG2000 showed no signal of toxicity even up to 200μg/ml. The uptake of SPIONS was detected using magnetization measurement, confocal and atomic force microscopy. SPION-PEG2000 presented the highest internalization capacity, which should be correlated with the mPEG chain size. The in vivo results suggested that SPION-PEG2000 administration in mice triggered liver and kidney injury. FROM THE CLINICAL EDITOR: The potential use of superparamagnetic iron oxide nanoparticles (SPIONS) in the clinical setting have been studied by many researchers. The authors synthesized two types of SPIONS here and investigated the physical properties and biological compatibility. The findings should provide more data on the design of SPIONS for clinical application in the future.