In the latest Paleocene an abrupt shift to more negative Δ13C values has been documented at numerous marine and terrestrial sites <Bralower et al., 1997; Cramer et al., 1999; Kaiho et al., 1996; Kennett and Stott, 1991; Koch et al., 1992; Stott et al., 1996; Thomas and Shackleton, 1996; Zachos et al., 1993>. This carbon isotope event (CIE) is coincident with oxygen isotope data that indicate warming of surface waters at high latitudes of nearly 4¿--6 ¿C <Kennett and Stott, 1991> and more moderate warming in the subtropics <Thomas et al., 1999>. Here we report 187Os/188Os isotope records from the North Atlantic and Indian Oceans which demonstrate a >10% increase in the 187Os/188Os ratio of seawater coincident with the late Paleocene CIE. This excursion to higher 187Os/188Os ratios is consistent with a global increase in weathering rates. The inference of increased chemical weathering during this interval of unusual warmth is significant because it provides empirical evidence supporting the operation of a feedback between chemical weathering rates and warm global climate, which acts to stabilize Earth's climate <Walker et al., 1981>. Estimates of the duration of late Paleocene CIE <Bains et al., 1999; Bralower et al., 1997; Norris and R¿hl, 1999; R¿hl et al., 2000> in conjunction with the Os isotope data imply that intensified chemical weathering in response to warm, humid climates can occur on timescales of 104--105 years. This interpretation requires that the late Palcocene thermal maximum Os isotope excursion be produced mainly by increased Os flux to the ocean rather than a transient excursion to higher 187Os/188Os ratios in river runoff. Although we argue that the former is more likelythan the latter, we cannot rule out significant changes in the 187Os/188Os ratio of rivers. ¿ 2001 American Geophysical Union