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In a 24-h period from 23 to 24 September 2009, a dust storm passed over Sydney, Australia that produced a red sky and reduced the visibility to a few metres. It was Sydney’s worst dust storm since 1942. During this period, the PM10 (particles measuring 10 mu m or less) value jumped from 50 to 11 800 mu g m(-3). The dust storm was sampled and its mineralogical and organic contents were analysed. Four major particle sizes (0.6, 4.5, 9.3 and 20 mm) were observed in the dust. A multimodal particle distribution indicated a long range of dust transport. Mineralogical analysis showed that the particles were mainly composed of crustal elemental oxides of Al and Si. The ratio of Al/Si was 0.39 and the organic content was 10.6 %, which was found to be enriched with humic-type substances. The high Al/Si ratio (>0.3) indicated that the dust originated from desert land whereas the high organic content indicated that the particles were also derived from eroded agricultural land. A fluorescence spectroscopic study on the organic matter at excitation and emission wavelengths of 245-265 and 330-350 nm indicated that biohazardous substances were unlikely to be present in the dust.