Abstract: Mass transfer behaviour of CO_2 absorption by BV solution has been studied in a modified wetted-wall column. The machanism and factors influencing mass transfer have been discussed, and mathematical model of mass transfer established. The concept of the critical liquid film thickness in the absorption involving fast pseudo-first order reaction was presented. It can convincingly explain the difference between mass transfer in an experimental absorber and that in an industrial packed column, and provide a theoretical foundation for intensifying the mass transfer equipment and understanding the mass transfer process.