Greigite (FeS1.34) was identified in the black magnetic fraction of a freeze-dried unoxidised estuarine sediment by X-ray diffraction and scanning electron microscopy with quantitative energy dispersive X-ray analysis. The greigite crystals were tabular, unlike the cubic morphology described in the literature, and ranged in size up to 1 μm wide and 0.2 μm thick. The stability of greigite was monitored by changes in peak intensities in X-ray diffractograms for a sample of greigite after exposure to a range of oxidising conditions. Moist greigite (with an approximate moisture content of 0.35 g/g) oxidised within hours under ambient conditions at 25°C and within minutes at 88°C. However, greigite was relatively stable if kept air-dry. The results indicate standard sample-handling and oven-drying procedures presently used for sulfidic sediments to minimise oxidation actually enhance greigite oxidation and suggest greigite may have an important role in the acidification of these sediments in the field.