Based on the monthly Î´(18)O value measured over a hydrology period in precipitation, runoff of five tributaries and the main lake of the Poyang Lake Basin, combined with hydrological and meteorological data, the characteristics of Î´(18)O in precipitation (Î´(18)OPPT) and runoff (Î´(18)OSUR) are discussed. The Î´(18)OPPT and Î´(18)OSUR values range from-2.75 to-14.12 â€° (annual mean value=-7.13 â€° ) and from-2.30 to-8.56 â€°, respectively. The seasonal variation of Î´(18)OPPT is controlled by the air mass circulation in this region, which is dominated by the Asian summer monsoon and the Siberian High during winter. The correlation between the wet seasonal averages of Î´(18)OSUR in runoff of the rivers and Î´(18)OPPT of precipitation at the corresponding stations shows that in the Poyang Lake catchment area the river water consists of 23% direct runoff (precipitation) and 77% base flow (shallow groundwater). This high proportion of groundwater in the river runoff points to the prevalence of wetland conditions in the Poyang Lake catchment during rainy season. Considering the oxygen isotopic composition of the main body of Poyang Lake, no isotopic enrichment relative to river inflow was found during the rainy season with maximum expansion of the lake. Thus, evaporation causing isotopic enrichment is a minor component of the lake water balance in the rainy period. During dry season, a slight isotopic enrichment has been observed, which suggests a certain evaporative loss of lake water in that period.