Long-term observations of the marine atmospheric boundary layer were performed by an eddy correlation system, which was set-up on a platform in the Baltic Sea. In this experiment the three-dimensional wind vector and the turbulent fluxes of momentum, sensible and latent heat and CO 2 were measured for one and a half years. Simultaneously the CO 2 partial pressure pCO 2 in surface water was measured by a submersible autonomous moored instrument for CO 2 at the platform in 7-m depth. The high-resolution eddy correlation measurements of the atmospheric CO 2 flux FCO 2 , together with the measurements of the CO 2 partial pressure differences between air and sea ΔpCO 2 led to a long-term data set which provided the possibility to investigate the parameterization of the CO 2 transfer velocity k as a function of 10-m wind speed u in a statistical manner. From half-hour mean CO 2 fluxes and CO 2 partial pressure differences, k was calculated using k=FCO 2 /(K 0 ΔpCO 2 ), with K 0 the CO 2 solubility. The half-hour mean data points, used for the determination of the k–u parameterization, show large scatter. However, assuming a linear, quadratic dependency the analysis yields: k 660 =0.365u 2 +0.46u (k at 20 °C and salinity 35 psu) with a correlation coefficient of r 2 =0.81. The large scatter indicates that the kinetics of the air–sea CO 2 transfer velocity is not only a function of the wind speed alone, but might also be controlled by other environmental parameters and mechanisms, such as sea state and surface coverage with surfactants.