The velocity distribution in Rayleigh convection caused by acetone volatilization in acetone-ethyl acetate binary system was observed in a vertical cross section of an initially quiescent liquid layer by utilizing particle image velocimetry. Obvious turbulent vortexes that were induced by Rayleigh convection appeared in the bulk liquid

and its statistic features indicated that Rayleigh convection became more intensewith the increase of Ra number and ReG number. Mass transfer coefficient was measured and the computed enhancement factor indicated that Rayleigh convection could promote the surface renewal of the liquid phase and intensify the interfacial mass transfer significantly. A method was proposed for the prediction of mass transfer coefficient based on the measured velocity vector

and the predicted mass transfer coefficients are in reasonable agreement with the experimental results.