The Permian-Triassic boundary records the most severe mass extinctions in Earth’s history. Siberian flood volcanism,the most profuse known such subaerial event, produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less. Analysis of Ar-40/Ar-39 data from two tuffs in southern China yielded a date of 250.0 +/- 0.2 million years ago for the Permian-Triassic boundary, which is comparable to the inception of main stage Siberian flood volcanism at 250.0 +/- 0.3 million years ago. Volcanogenic sulfate aerosols and the dynamic effects of the Siberian plume likely contributed to environmental extrema that led to the mass extinctions.