The Chicxulub impact crater in Mexico is the site of the impact purported to have caused mass extinctions at the Cretaceous/Tertiary (K/T) boundary. 2-D hydrocode modeling of the impact, coupled with studies of the impact site geology, indicate that between 0.4 and 7.0 × 10 17 g of sulfur were vaporized by the impact into anhydrite target rocks. A small portion of the sulfur was released as SO 3 or SO 4, which converted rapidly into H 2SO 4 aerosol and fell as acid rain. A radiative transfer model, coupled with a model of coagulation indicates that the aerosol prolonged the initial blackout period caused by impact dust only if the aerosol contained impurities. A larger portion of sulfur was released as SO 2, which converted to aerosol slowly, due to the rate-limiting oxidation of SO 2. Our radiative transfer calculations, combined with rates of acid production, coagulation, and diffusion indicate that solar transmission was reduced to 10-20% of normal for a period of 8-13 yr. This reduction produced a climate forcing (cooling) of -300 Wm -2, which far exceeded the +8 Wm -2 greenhouse warming, caused by the CO 2 released through the vaporization of carbonates, and therefore produced a decade of freezing and near-freezing temperatures. Several decades of moderate warming followed the decade of severe cooling due to the long residence time of CO 2. The prolonged impact winter may have been a major cause of the K/T extinctions.