Stable isotope compositions of present day precipitation in the 70¿E--160¿W longitude range reveal a strong correlation with moisture source and transport patterns and are not correlated with the amount of precipitation, contrary to existing interpretations. Spatially and temporally variable moisture sources and cyclonic circulation result in more depleted isotope values (δ18O ~ -5 to -7?) in the 100¿E--170¿E range. Oceanic moisture with minimal continental loss or input results in comparatively enriched δ18O values (~-2 to -4?) in the 70¿E--100¿E range and east of 170¿E. Isotope compositions of dated groundwater archives from the Philippines, Vietnam, Thailand and Bangladesh, reflecting past isotopic composition of precipitation, indicate that this distinction in moisture regimes was preserved during the last glacial period and strongly suggest that there was no significant shift in the summer Inter-Tropical Convergence Zone (ITCZ) in this region.