Analyses of total carbon dioxide (TCO2 ) and titration alkalinity (TA) in the mixed-layer was performed approximately bimonthly at the SEATS time series site (18°15'N, 115°35'E) in the northern South China Sea (SCS) from March 2002 to April 2003. These measurements and the calculated-fCO2 were then used to document their seasonal variations and to estimate the seasonal air-sea flux of CO2 during the observed period at the site. Results show that the normalized TCO2 ( NTCO2 = TCO2 x 35/S) fluctuates seasonally between ~1972 and ~1997 µmol kg-1 , with the highest value in winter. The decline of NTCO2 in spring-summer mainly results from in situ biological utilization, while the resurgence of NTCO2 in fall-winter is due to entrainment of the TCO2-rich subsurface waters from below. TA varies from ~2190 to ~2220 µmol kg-1 in tandem with salinity, suggesting the prime control of physical processes. fCO2 increases progressively from spring to summer, reaches a maximum in July (~382 µatm) decreases from fall to winter to a minimum (~347 µatm) in January with an amplitude of ~35 µatm. The seasonal variability of fCO2 is in phase with temperature changes but is inversely correlated with the fluctuation of NTCO2 , suggesting that the fCO2 seasonality is primarily controlled by temperature changes, though other factors have compensated partially to yield the observed low amplitude of its variability. The sea-to-air CO2 fluxes for spring, summer, fall and winter are estimated to range from 0.00 ± 0.01 to -0.02 ± 0.05, +0.03 ± 0.01 to +0.23 ± 0.06, +0.18 ± 0.10 to +0.45 ± 0.25, and -0.62 ± 0.20 to -1.42 ± 0.46 molC m-2 year-1 , respectively. Throughout the year, the annual flux is calculated to be -0.11 ± 0.08 ~ -0.23 ± 0.18 molC m-2 year-1 during the observed period. Furthermore, although there is a drawdown of NTCO2 of ~25 µmol kg-1 from winter to summer, which implies a net community production of 6.80 ± 0.77 mmolC m-2 year-1 in the mixed layer at the SEATS site, there is no corresponding change of nitrate observed, suggesting other sources of nitrogen required to sustain the new production.