Concentration distribution, sea-to-air flux, photoproduction of carbon monoxide (CO) in the surface seawater and atmospheric CO mixing ratio were measured in the Yellow Sea and the Bohai Sea. A headspace analysis system was used for CO measurement. The concentrations of CO in the surface seawater ranged from 0.19 to 3.57 nmol x L(-1), with an average of 1.24 nmol x L(-1) ( SD = 0.79, n = 69). Overall, the concentrations of CO displayed a decreasing trend from the coast to the offshore stations and followed diurnal variations after classifying and averaging the CO concentrations according to sampling and analysing time in the unit of one hour, with the maximum values in midnoon which was 10 folds higher than the minimum values in predawn. Atmospheric CO mixing ratios varied from 215 x 10(-9) to 850 x 10(-9), with an average of 414 x 10(-9) (SD = 140 x 10(-9), n = 69), due to obvious terrestrial input. The supersaturation factors of CO varied from 0.42-18.90, with an average of 3.61 (SD = 2.99, n = 69), indicating that the Yellow Sea and the Bohai Sea was a net source of atmospheric CO. The average sea-to-air fluxes of CO from the Yellow Sea and the Bohai Sea were estimated to be (1.22 +/- 1.70) micromol x (m2 x d)(-1) by the LM86 equation and (2.13 +/- 2.91) micromol x (m2 x d)(-1) by the W92 equation, respectively. Using SMARTS2 spectral irradiance model to estimate CO production, the photoproduction rate of CO was 54.60 micromol x (m2 x d)(-1) and the photoproduction of carbon in the Yellow Sea and the Bohai Sea was 26.95 x 10(9) g in spring. The photoproduction rate of CO was 25-50 times higher than the sea-to-air flux, suggesting that most part of CO was removed by microbial process in the surface seawater.