A high-resolution study of clay mineralogy and major element geochemistry has been conducted on high sedimentation rate cores (MD01-2393 and MD97-2150) collected off the Mekong River mouth in the southwestern South China Sea in order to reconstruct the erosional and weathering history of the Mekong Basin. The chronology is based upon planktonic foraminiferal oxygen isotope records combined with carbonate stratigraphy. Clay minerals and major element results suggest that the Mekong River is the major sedimentary source over the past 190 kyr for both cores. Illite and chlorite were derived mainly from the eastern Tibetan Plateau. Kaolinite was derived mainly from active erosion of inherited clays from reworked sediments in the middle part of the Mekong Basin. Smectites originated mainly through chemical weathering of parent aluminosilicate and ferromagnesian silicate under warm and humid conditions in the middle to lower parts of the Mekong Basin. Smectites/(illite+chlorite) and smectites/kaolinite ratios coupled to K2O/SiO2 and Al2O3/SiO2 ratios allow us to reconstruct a history of chemical weathering versus physical erosion. Good correlations between those ratios suggest monsoon-controlled weathering and erosion over the eastern Tibetan Plateau and the Mekong Basin. Variations with time scales of 0.3-0.6kyr for the last 70kyr and 1-2kyr for 70-190kyr ago in the chemical weathering/erosional history are strongly related to the East Asian monsoon evolution, which is itself related to the summer solar radiation calculated for the Northern Hemisphere.