Composites made of iron and Cr doped magnetite are obtained by precipitation from the metallic chlorides salts into a KOH solution heated under reflux. A KOH concentration of 14 mol l⁻¹ is required. The composition of the samples is determined by X-ray diffraction, TG measurements, chemical analysis and Mössbauer spectrometry. The higher the chromium content, the smaller the lattice parameter and the lower the metal ratio. For Cr/Fe ≤ 0.1, the chemical formula of the composites can be written Fe_A/(Fe$^{\nu+}_{x}$Cr$^{3+}_{y}\square_{3-(x+y)}$O$_{4})_{1-{\rm A}}$, $0\le y\le0.23$. We show that only in-field Mössbauer spectrometry at low temperature is suitable to estimate unambiguously both the proportions of octahedral and tetrahedral iron sites in spinel and the valence of delocalised iron state, if any. A peculiar A-site occupation by either vacancies or Cr³⁺ ions is observed. This is probably due to the low temperature synthesis. The two phases, metal and spinel are found to be intimately mixed within the nanometer sized particles. For Cr/Fe ratios higher than 0.1, hydroxide phase is also present.