Two Permian-Triassic boundary (PTB) sections from south China provide insights regarding the origin of negative excursions in carbonate and organic carbon isotope records associated with the largest mass extinction in Earth history. Xiakou, a carbonate ramp section, exhibits δ13Ccarb excursions of up to –2‰, and Xinmin, a deep-shelf section, exhibits δ13Corg excursions of up to –6‰. In both sections, these excursions are associated with volcanic ash layers, and excursion size scales with ash layer thickness. These relationships document the direct influence of volcanism on the Earth-surface carbon cycle during the PTB crisis. Previous studies of ash layers in south China PTB sections have invoked a source in regional subduction-zone volcanism in the eastern Tethys, but our analysis suggests that these ash layers may represent distal deposits from large-scale explosive eruptions of the Siberian Traps. If confirmed by further investigation, this hypothesis would have important implications both for kill mechanisms during the PTB crisis as well as for refinement of the eruption history of the Siberian Traps.