Monthly samples of riverine water were collected and analyzed for the concentrations of major ions (Ca2+, Mg2+, K+, Na+, HCO3-, SO42-, Cl-, NO3-), dissolved silicon, and total dissolved solids (TDS) at Wuzhou hydrological station located between the middle and lower reaches of the Xijiang River (XJR) from March 2005 to April 2006. More frequent sampling and analysis were carried out during the catastrophic flooding in June 2005. Stoichiometric analysis was applied for tracing sources of major ions and estimating CO2 consumption from the chemical weathering of rocks. The results demonstrate that the chemical weathering of carbonate and silicate rocks within the drainage basin is the main source of the dissolved chemical substances in the XJR. Some 81.20% of the riverine cations originated from the chemical weathering processes induced by carbonic acid, 11.32% by sulfuric acid, and the other 7.48% from the dissolution of gypsum and precipitates of sea salts within the drainage basin. The CO2 flux consumed by the rock chemical weathering within the XJR basin is 2.37 x 10(11) mol y(-1), of which 0.64 x 10(11) mol y(-1) results from silicate rock chemical weathering, and 1.73 x 10(11) mol y(-1) results from carbonate rock chemical weathering. The CO2 consumption comprises 0.38 x 10(11) mol during the 9-d catastrophic flooding. The CO2 consumption from rock chemical weathering in humid subtropical zones regulates atmospheric CO2 level and constitutes a significant part of the global carbon budget. The carbon sink potential of rock chemical weathering processes in the humid subtropical zones deserves extra attention.