Interacting superparamagnetic iron(II) oxide nanoparticles (NPs) with sizes of 5.3 1.6 nm were prepared by simple decomposition of [Fe(COT)(2)] (COT = 1,3,5,7-cyclooctatetraene) with 5 bar of H-2 in 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMINTf2) ionic liquid (IL). The static and dynamic magnetic characterization revealed a superparamagnetic behavior with weak dipolar interactions of these NPs. In situ structural studies by X-ray absorption spectroscopy demonstrated that they consist of nanostructured FeO. This approach is an appropriate method to prepare and stabilize nanostructured FeO particles, where the presence of an IL proved to be fundamental to suppress the aggregation and usual overoxidation of the FeO NPs.