The crystallochemical characterization of magnetic spinels prepared from aqueous solution has been studied primarily by electron microscopy and 57Fe Mossbauer spectroscopy. Two synthetic routes have been investigated : method 1 ; partial oxidation of Fe" solutions in the presence of NO; at 100 C , and method 2; reaction of hydrated Ferrr oxide (ferrihydrite) with Fe" ions at room temperature, pH = 7, 8 and 9. Both routes gave small (10-60 nm) irregular magnetite (Fe,O,) particles that were readily oxidised to maghemite (y-Fe,O,). The reaction proceeded via a green-rust intermediate and anions such as inorganic phosphate (Pi) and SO,,- 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 mol %) 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.