Laboratory measurements of gas-transfer velocity and free-surface hydrodynamics were made for oscillating grid-stirred turbulence. Air-water gas-transfer velocities were determined from the waterside dissolved oxygen mass balance, and an innovative digital particle image velocimetry technique was used to measure the two-dimensional free-surface flow field. The impact of adventitious and purposely introduced surfactant films on free-surface turbulence and gas-transfer rates was explored quantitatively. As expected, bulk turbulence was unable to provide a unique relationship for the gas-transfer rates observed, owing predominantly to surfactant effects. However, the surface divergence computed from the free-surface velocity field was capable of reconciling the gas-transfer data. This is possible evidence confirming that the free-surface divergence is an important process involved in interfacial gas transport. A relationship for the gas-transfer velocity in terms of the surface divergence and the surface pressure is presented that successfully relates the interactions between surface renewal, surfactants, and gas transfer. (C) 2003 Elsevier Ltd. All rights reserved.