The mass transfer process under CO2-water Taylor flow was experimentally investigated in circular capillaries with different lengths. The measured volumetric mass transfer coefficient k(L)a was found to reduce with the increase of mass transfer time under the same operational conditions. With computational fluid dynamics simulations, the instantaneous k(L)a values decreased sharply at the initial stage of the mass transfer process. The effects of numerous experimental parameters on separated contribution of k(L)a were investigated for different dimensionless mass transfer times. The instantaneous k(L)a values for arbitary transfer times could be calculated and agreed well with experimental data.