Most mass extinctions are linked with carbon isotope excursions, implying that biotic crises are coupled with changes in the global carbon cycle. The isotopic evolution during the end-Triassic extinction is far less documented than that for the other major Phanerozoic extinctions. Here we report a sharp and short-lived −3.5‰ excursion in carbon isotope values for carbonate (δ13Ccarb) corresponding to a −2‰ excursion in the isotopic composition of marine organic matter (δ13Corg) and other geochemical changes from the topmost Triassic in the Csovár section in Hungary. The Triassic- Jurassic boundary is defined by ammonoid and conodont biostratigraphy in a marine limestone sequence. A decline in primary productivity, release of methane through dissociation of gas hydrates, or a combination of the two may account for the correlative biotic and isotopic events.