Ethanol gas is added to CO2 in order to enhance the absorption of CO2 into water in a bubble column. The mass transfer coefficient exhibits a hump form with increasing ethanol concentration in the gas phase. Such a phenomenon is interpreted by the coeffect of the local Marangoni effect and the negative Rayleigh effect. This assumption is justified by a particle image velocimetry experiment. A model based on Higbie's penetration theory and Kolmogorov's isotropic turbulence theory to calculate the mass transfer coefficient for the absorption process is correlated in terms of the Rayleigh number and the Marangoni number and is shown to be able to reproduce the hump-form behavior of the mass transfer coefficient.