Abstract This paper reports some new results concerning the structure of the concentration field and the mechanisms of mass transfer at flat, turbulent, gas–liquid interfaces, especially in the limit of high-Schmidt numbers. The problem is investigated using the Large Eddy Simulation technique applied to two different kinds of interfaces, namely a sheared-driven interface and a shear-free surface. Statistical results show that the concentration field is greatly affected by the structure of the turbulence in the diffusive sublayer located just below the interface. When no shear is applied on the interface, the instantaneous picture of the interfacial structures reveals that the concentration field is organized in large patches that mirror the upwelling structures coming from the core of the flow. In contrast at a sheared interface the concentration field is organized in elongated structures of much smaller spanwise extent. The shape of these structures is found to result from the combined effect of advection by the streamwise velocities which are themselves organized in streaks, and of the vertical motions induced by bursting events. For both kinds of interfaces, it is found that the concentration fluctuations are closely related to the horizontal turbulent motions that stretch the interface. The results of the simulations are used to check several mass transfer models. In accordance with instantaneous visualizations and statistical properties of the concentration field, the model of McCready et al. (AIChE J.32 (1986) 1108–1115) involving the surface divergence appears to be the most suitable for predicting the mass transfer coefficient through gas–liquid interfaces.