The crystallochemical characterization of magnetic spinels prepared from aqueous solution has been studied primarily by electron microscopy and 57Fe Mössbauer spectroscopy. Two synthetic routes have been investigated: method 1; partial oxidation of FeII solutions in the presence of NO–3 at 100 °C, and method 2; reaction of hydrated FeIII oxide (ferrihydrite) with FeII ions at room temperature, pH = 7, 8 and 9. Both routes gave small (10–60 nm) irregular magnetite (Fe3O4) particles that were readily oxidised to maghemite (γ-Fe2O3). The reaction proceeded via a green-rust intermediate and anions such as inorganic phosphate (Pi) and SO2–4 reduced the rate of transformation. Spinel formation at room temperature was severely inhibited by Pi levels of 1 mol %, whereas 20–30 mol % Pi was required for retardation of crystallization at 100 °C. Intermediate levels (5–10 mole %) resulted in morphological enhancement of the octahedral crystal habit. A similar effect was observed in the presence of SO2–4 and at neutral pH in method 2.