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The age and timing of the Permian-Triassic mass extinction have been difficult to determine because zircon populations from the type sections are typically affected by pervasive Lead loss and contamination by indistinguishable older xenocrysts. Zircons from nine ash beds within the Shangsi and Meishan sections (China), pretreated by annealing followed by partial attack with hydrofluoric acid, result in suites of consistent and concordant uranium/lead (U/Pb) ages, eliminating the effects of lead Loss. The U/Pb age of the main pulse of the extinction is 252.6 ± 0.2 million years, synchronous with the Siberian flood volcanism, and it occurred within the quoted uncertainty., Despite intensive study, the cause and timing of events associated with the Permian-Triassic (P-T) extinction--the most profound such extinction known over the history of life on Earth--remain uncertain. Most scenarios [...]