We have analyzed clay minerals and oxygen isotopes over the Eocene-Oligocene climate transition (33.8–32.5 Ma) in Ocean Drilling Program Site 689, Maud Rise, Antarctica. Distinct changes in clay mineral assemblages suggest major instability in the East Antarctic climate for 0.7 m.y. during this time of general expansion of the Antarctic cryosphere and cooling of the Southern Ocean. Increased illite abundance reflects enhanced physical weathering associated with cryospheric development. Nevertheless, continued dominance of smectite indicates that chemical weathering continued to prevail during the early Oligocene. Weathering was much more intense than in modern deglaciated areas. The clay mineral data support evidence from marine sediments suggesting that continental and marine climatic conditions during the early Oligocene were intermediate between relative Eocene warmth and intense Neogene cold. The clay mineral variations during the transition reflect major changes in continental precipitation and related continental ice accumulation.