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The onset of the Asian summer monsoon is noticeably controversial, spatially and temporally. The stable oxygen isotope delta O-18 in precipitation has long been used to trace water vapor source, particularly to capture the summer monsoon precipitation signal. The abrupt decrease of precipitation delta O-18 in the Asian summer monsoon region closely corresponds to the summer monsoon onset. Two stations have therefore been set up at Guangzhou and Lulang in the East Asian summer monsoon domain to clarify the summer monsoon onset dates. Event-based precipitation delta O-18 during 2007/08 is much lower at Lulang than at Guangzhou and is attributable mainly to the altitude effect offset by different isotopic compositions in marine moisture sources. The earlier appearance of low delta O-18(wt) at Lulang than at Guangzhou confirms the earlier summer monsoon onset in the Bay of Bengal. Isotopically identified summer monsoon evolutions from precipitation delta O-18 at both stations are verifiable with NCEP-NCAR reanalysis data, indicating that precipitation delta O-18 offers an alternative approach to studying the summer monsoon circulation from precipitation delta O-18.