Magnetite (Fe3O4) nanoparticles are important as contrast agents in magnetic resonance imaging or for magnetic drug targeting. Such particles can be made by different ways of synthesis, but depending on their size they tend to oxidise to maghemite (γ-Fe2O3), which is often less desirable because of its lower magnetisation. Mössbauer spectroscopy is well suited for determining the relative amounts of the two iron oxides in a sample. When measured at 4.2 K the nanoparticles typically exhibit well-defined but complicated hyperfine spectra that may present some problems of evaluation, but eventually yield reliable results for the degree of oxdation. At room temperature, however, particles smaller than about 15 nm are affected by superparamagnetic relaxation, which renders Mössbauer spectroscopy useless for their characterisation. To characterise magnetic nanoparticles even at room temperature, we designed an arrangement of permanent magnets to apply an external magnetic field of about 0.7 T to the Mössbauer absorbers. This has been found to be sufficient to give rise to magnetically split Mössbauer spectra that allow a distinction between magnetite and maghemite and to determine their relative amounts in a sample.