The uptake of atmospheric carbon dioxide (CO 2 ) into the mid-latitudes of the North Atlantic Ocean through the production of wintertime Sub-Tropical Mode Water (STMW) also known as Eighteen Degree Water (EDW) is poorly quantified and constrained. Nonetheless, it has been proposed that the EDW could serve as an important short-term sink of anthropogenic CO 2 . The objective of the present investigation was to determine sea–air CO 2 gas exchange rates and seawater CO 2 dynamics during wintertime formation of EDW in the North Atlantic Ocean. During 2006 and 2007, several research cruises were undertaken as part of the CLIMODE project across the northwest Atlantic Ocean with the intent to study the pre-conditioning, formation, and the evolution of EDW. Sea–air CO 2 exchange rates were calculated based on measurements of atmospheric pCO 2 , surface seawater pCO 2 and wind speed with positive values denoting a net flux from the surface ocean to the atmosphere. Average sea–air CO 2 flux calculated along cruise tracks in the formation region equaled −18±6mmol CO 2 m −2 d −1 and −14±9mmol CO 2 m −2 d −1 in January of 2006 and March of 2007, respectively. Average sea–air CO 2 flux in newly formed outcropping EDW in February and March of 2007 equaled −28±10mmol CO 2 m −2 d −1 . These estimates exceeded previous flux estimates in this region by 40–185%. The magnitude of CO 2 flux was mainly controlled by the observed variability in wind speed and ΔpCO 2 with smaller changes owing to variability in sea surface temperature. Small but statistically significant difference (4.1±2.6μmolkg −1 ) in dissolved inorganic carbon (DIC) was observed in two occurrences of newly formed EDW in February and March of 2007. This difference was explained either by differences in the relative contribution from different water masses involved in the initial formation process of EDW or temporal changes owing to sea–air CO 2 exchange (∼25%) and vertical and/or lateral mixing (∼75%) with water masses high in DIC from the cold side of the Gulf Stream and/or from below the permanent thermocline. Based on the present estimate of sea–air CO 2 flux in newly formed EDW and a formation rate of 9.3Svy (Sverdrup year=10 6 m 3 s −1 flow sustained for 1 year), CO 2 uptake by newly formed EDW may constitute 3–6% of the total North Atlantic CO 2 sink. However, advection of surface waters that carry an elevated burden of anthropogenic CO 2 that are transported to the formation region and transformed to mode water may contribute additional CO 2 to the total net uptake and sequestration of anthropogenic CO 2 to the ocean interior.