『Abstract
　Clay mineralogy and major-element geochemistry were investigated on 58 surface sediment samples collected in 27 rivers draining Malay Peninsula, Borneo, and Sumatra to assess the chemical weathering process and its controlling factors in tropical Southeast Asia. The clay mineral assemblages show high kaolinite contents in Malay Peninsula (average 80%), Sumatra (58-78%), and South Borneo (41-55%), high illite contents in North Borneo (47-77%), and moderate smectite contents in Sumatra (6-29%). Intensive chemical weathering is identified in all three investigated regions from both clay mineralogical and geochemical (bulk and clay-fraction) results regardless of their various lithological and tectonic settings. Relatively, the chemical weathering intensity increases gradually from North Borneo to South Borneo, and further to Malay Peninsula and Sumatra. Our results suggest that the East Asian-Australian monsoon climatic condition with constant warm temperature and abundant precipitation throughout the year is the principal forcing factor on the chemical weathering, and the tectonic activity including mountain uplift, earthquakes, and volcanic eruptions and their specific lithological characters is only a subordinate factor on the intensity of chemical weathering in tropical Southeast Asia.

Keywords: Clay minerals; Major elements; Chemical weathering; Malay Peninsula; Borneo; Sumatra』

1. Introduction
2. hydrographic and geological settings
3. Sampling and analytical methods
4. Results
　4.1. Clay minerals
　4.2. Major elements
5. Discussion
　5.1. Clay-mineralogical changes
　5.2. Intensity of chemical weathering
6. Conclusions
Acknowledgments
Appendix A. Supplementary data
References