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Earth's climate transitioned from the greenhouse conditions of the Eocene to the icehouse of the Oligocene similar to 34 million years ago. Global cooling was triggered by a decrease in atmospheric pCO(2) and is associated with the first appearance of large, permanent ice-sheets on Antarctica. The Eocene-Oligocene transition (EOT) is marked by a large two-step increase in the delta O-18 of benthic foraminifera, reflecting a combination of cooler global temperatures and continental ice growth. We measured the hydrogen isotope composition of hydrated volcanic glasses deposited in Patagonia during the EOT to reconstruct stable isotope hydrology and associated changes in temperature during this critical climate transition. These data show changes in stable hydrogen isotope ratios that are consistent with Patagonian terrestrial mean annual temperature decreasing by similar to 5 degrees C during the EOT before returning to 2 degrees C below pre-EOT values by the Early Oligocene. This is approximately synchronous with Northern Hemisphere cooling during the EOT, and indicates close coupling of temperature and atmospheric pCO(2). (C) 2019 Elsevier B.V. All rights reserved.