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Tapered element oscillating microbalances (TEOMs) are used in the UK Automatic Monitoring Network for the continuous measurement of ambient airborne particles. Used TEOM filters from Swansea, Cardiff and Pembroke were examined under high-resolution field emission scanning electron microscopy (FESEM). Clusters of calcium sulphate crystals, gypsum (CaSO4 . 2H(2)O) and anhydrite (CaSO4) were abundant on spring and summer filters, and not present on autumn and winter filters. From textural considerations, the sulphates must have crystallised on the filter surfaces, either by dissolution and recrystallisation of CaSO4 collected as particles, or by direct precipitation from saline water collected on the filters; in much the same way as the formation of ’desert roses’ by the evaporation of saline pore waters in desert sands. The proposed mechanism for the formation of these crystals has two important implications. Firstly, if the filters are episodically saturated with water, then on occasion the recorded masses will consist of both particles plus water, causing errors in the results of continuous monitoring; an important consideration for epidemiological studies based on TEOM data. Secondly, past toxicological experiments undertaken on TEOM-derived ’PM10’ may have investigated material containing a significant component of in situ formed crystals, rather than the original PM10. (C) 2001 Elsevier Science Ltd. All rights reserved.