Mixed-valent Fe(II),Fe(III)-layered hydroxide, known as green rust, was synthesized from slightly basic, sodium sulphate solutions in an oxygen-free glove box. Solution conditions were monitored with pH and Eh electrodes and optimized to ensure a pure sulphate green-rust phase. The solid was characterised usingMo¨ssbauer spectroscopy, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The composition of the solution from which the green rust precipitated was established by mass and absorption spectroscopy. The sulphate form of green rust is composed of brucite-like layers with Fe(II) and Fe(III) in an ordered distribution. The interlayers contain sulphate, water and sodium in an arrangement characteristic for the nikischerite group. The crystal structure is highly disordered by stacking faults. The composition, formula and crystallographic parameters are: NaFe(II)₆Fe(III)₃(SO₄)₂(OH)₁₈•12H₂O, space group P-3, a = 9.528(6)Å, c = 10.968(8)Å andZ = 1. Green rust sodium sulphate, GR_Na,SO4, crystallizes in thin, hexagonal plates. Particles range from less than 50 nm to 2 µm in diameter and are 40 nm thick or less. The material is redox active and reaction rates are fast. Extremely small particle size and high surface area contribute to rapid oxidation, transforming green rust to an Fe(III)-phase within minutes.