Aluminium citrate has been identified as the major component of a sediment formed as a result of microbial degradation of a jet fuel at the fuel-water interface. The structure of the sediment was determined using a combination of wet chemistry, elemental analysis, pyrolysis-gas chromatography-mass spectrometry, gas chromatography-mass spectrometry and infrared spectroscopy. The sediment sample was insoluble in organic solvents, but soluble in water and dilute base. The methylated derivatives of the sediment were organic solvent soluble and amenable to study using standard techniques. Citric acid and related carboxylic acid compounds were shown to be the principal organic compounds present. The similarity between the infrared spectrum of the sediment and that of aluminium citrate suggested that this species was the dominant component of the sediment. It is proposed that the most likely source of the sediment was citric acid produced by microbiological activity at the fuel-water interface which subsequently reacted with the aluminium leached from clays.