The Xinanjiang Reservoir (E118 degrees 42'-E118 degrees 59', N29 degrees 28'-N29 degrees 58'), has a surface area of 567 km(2), with a mean water depth of 34 m, and is in an oligotrophic state at present. In this study, five cruises were carried out in April, June, August and November 2010, and January 2011, to understand the seasonal variation of CO2 emission and to estimate the annual diffusion flux of CO2 from this reservoir. The partial pressure of CO2 (pCO(2)) in surface water was determined using a continuous measurement system (equilibrator-infrared system) from upstream to central reservoir, and pCO(2) along the water column in central reservoir and in downstream of the dam was also determined. Results showed that pCO(2) in reservoir surface water varied significantly in different seasons, ranging from 5 gatm in August to 1700 mu atm in January; while in downstream of the dam, it ranged from 1400 mu atm in April to 3800 gatm in August. Along the water column in central reservoir, pCO(2) also showed significantly variations seasonally, and a water mass with quite high pCO(2) (>4000 mu atm) formed below water depth of 20 m in warm seasons. The highest diffusion flux of CO2 (F-CO2) appeared in January, with values of 20.9 mmol IT1-2 d(-1) at river reach and 25.2 mmol m(-2) d(-1) at central reservoir, while the lowest values of F-CO2 were -10.0 at river reach and -10.8 mmol rr1-2 (1-1 at central reservoir in August. Downstream of the dam had quite high positive F-CO2 during the whole year, with a range of 129.1 in April to 381.5 mmol m(-2) d(-1) in August. In total, about 88.9 kton a(-1) CO2 was emitted to the atmosphere from the Xinanjiang Reservoir surface, while 39% of it (35.1 kton a-1) was absorbed by surface water in warm seasons. Downstream and turbine had a comparable CO2 outgas flux of 61.5 kton a(-1). When taking the whole reservoir surface, turbine and downstream into account, the Xinanjiang Reservoir system had a net CO2 emission flux of 115.3 kton a(-1)(C) 2014 Elsevier Ltd. All rights reserved.