Mass transfer across the gas-water interface is important in many fields, but the present understanding of the scalar transport as mediated by the complex near-surface turbulence is still far from complete. In this work, an innovative particle image velocimetry (PIV)-based measurement method was used to investigate the near surface turbulence. Turbulence measurement with respect to the fluctuating interface was performed and the gradient of the vertical fluctuating velocity (Hanratty's beta) deemed as a critical controlling parameter was quantified. Several distinctly different but typical flow conditions were investigated and the associated mass-transfer velocity was measured. Based on these experimental works, an empirical relation, very similar to that proposed earlier by Law and Khoo, relating the mass transport across the turbulent interface to the turbulence parameter beta, was determined in the midst of vastly different flow conditions where turbulence is induced simultaneously from above and beneath the gas-water interface. (c) 2006 American Institute of Chemical Engineers.