Magnetic nanoparticles (MNPs) of pure magnetite (Fe3O4) were prepared in an aqueous solution (sample M-I) and in a water-ethyl alcohol mixture (sample M-II) by the co-precipitation method. The structure and magnetic properties of both samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), magnetic (M-H) and microwave measurements (FMR). The mean average particle diameter and particle size distribution was evaluated by the Dynamic Light Scattering (DLS) and Brunauer- Emmett-Teller techniques (BET). The Quantitative chemical analysis of iron was performed by Inductively Coupled Plasma (ICP)- Atomic Emission Spectroscopy (AES) technique. The MNPs prepared in aqueous solution show a higher grain than those prepared in the water-ethyl alcohol mixture. The type of phase structure in both cases can be defined as "defective spinel". The shape of the majority of M-I MNPs is octahedral. The shape of the majority of M-II MNPs is cubic. The specific surface area of MNPs was as high as 14.4 m(2)/g for M-I sample and 77.8 m(2)/g for sample M-II. The obtained saturation magnetization values of 75 emu/g (M-I) and 68 emu/g (M-II) are consistent with expected values for magnetite MNPs of observed sizes. Ferromagnetic resonance (FMR) measurements confirmed that MNPs of both types are magnetically homogeneous materials. FMR lines' position and line widths can be understood by invoking the local dipolar fields, deviations from sphericity, magnetocrystalline anisotropy and stresses. M-I sample shows sizeable zero field microwave absorption which is absent in the M-II case. The differences in microwave behaviour of M-I and M-II MNPs can be used in the design of microwave radiation absorbing multilayers. (C) 2015 Elsevier B.V. All rights reserved.