Iron oxide nanoparticles are available in two common phases, namely magnetite (Fe 3 O 4) and maghemite (Fe 2 O 3). Upon exposure to oxygen atoms, the magnetite phase readily oxidizes into the maghemite phase with the partial conversion of ferrous ions into ferric ions. We report on the approach to determine the ratio of magnetite and maghemite in iron oxide nanoparticles synthesized by the wet chemical route. X-ray diffraction studies and transmission electron microscopy observations confirmed the formation of highly crystalline nanoparticles of size (7 ± 2) nm. The average particle size is in the magnetic single-domain range suitable for the superparamagnetic behavior. The Mössbauer spectrum of the sample is composed of two six-line patterns in perfect agreement with the theoretically predicted model. The extracted Mössbauer parameters show contribution of two phases accounting for 47% magnetite and 53% maghemite. The hysteresis loops of the iron nanoparticles demonstrated the "S-shaped" pattern with negligible coercivity and remanence magnetization. This result reveals a promising method to synthesize and characterize magnetic nanoparticles of uniform size with a potential for biomedical applications.