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RATIONALE Fulgurites are glassy crusts or hollow glassy tubes formed by the impact of a lightning strike on a target material on the Earth’s surface. The oxygen isotopic composition of fulgurites has never been measured and, consequently, it is unknown whether or not isotopic fractionations take place between the target material and the fulgurite glass during the lightning event which is an excellent natural example of extremely fast melting process. METHODS Following well-established procedures (high-temperature reaction of the fulgurite material with BrF5, conversion into CO2 of the evolved O2 and measurement of the18O/16O ratio on a Finnigan Delta S mass spectrometer) we measured for the first time the oxygen isotopic composition of sets of fulgurites coming from various locations on the Earth’s surface. RESULTS The range of isotopic values is quite large, probably reflecting the oxygen isotopic values of the target materials. In the case of fulgurites from the Sahara Desert the isotopic values obtained from the bulk material, quartz crystals sticking to the fulgurite body, tiny samples of loose sand coming from fulgurite bubbles, and sand samples collected near the fulgurites, are very close to one another. CONCLUSIONS Although we do not have indisputable evidence, we conclude that, at least in the case of oxygen, the fusion process of the material struck by lightning, as well as all the extremely fast high-temperature fusion processes, probably take place without any isotopic fractionation effect. Copyright (C) 2012 John Wiley & Sons, Ltd.