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Abstract: The concentration of gaseous carbon dioxide (CO2) in surface seawater is a fundamental control on the CO2 flux between the ocean and atmosphere. However, the concentration gradient in the aqueous mass boundary layer determines the magnitude and direction of the flux. The gradients of CO2 in the aqueous mass boundary layer cannot be measured directly and are usually inferred from partial pressures or fugacities of CO2 (f CO2 ) in the air and water. In addition to the f CO2 , the temperatures at the top and bottom of the aqueous mass boundary layer must be known to determine the thermodynamic driving force of CO2 gas transfer. Expressing the gradient in terms of the aqueous CO2 concentration, [CO2aq], also avoids some conceptual ambiguities. In particular, expressing the CO2 as a fugacity, which is defined relative to the gas phase, when the gas exchange rate is controlled in the aqueous mass boundary layer often leads to errors in interpretation with respect to changes in boundary layer temperature. As a result the enhanced CO2 flux caused by the cool skin effect appears to be overestimated. Apart from the difficulties estimating the temperature at the top and bottom of the aqueous mass boundary layer, the temperature dependence of solubility and fugacity of CO2 is uncertain to the degree that it can bias air–sea CO2 flux estimates. The CO2aq at the surface, [CO2aq0], is at equilibrium with the atmospheric CO2 level. As [CO2aq0] is strongly temperature dependent, it will be significantly higher at high latitude compared to low latitude, while atmospheric CO2 levels show much less of a gradient. [Copyright &y& Elsevier]