Monoethanol amine (MEA) aqueous solution is one of the most widely used absorbents for CO2 in industry. The mass transfer behavior of CO2 absorption into MEA aqueous solution in a microreactor was investigated experimentally. Liquid side volumetric mass transfer coefficients (k(L)a) of CO2 were measured in horizontal microreactor with rectangular cross section. The influences of gas and liquid phase flow rates on k(L)a for Taylor and annular flow were primarily studied. The concentration of CO2 in liquid phase at the outlet of microreactor was obtained by titration method and then k(L)a was calculated. Results indicated that: under the same liquid side flow rate, k(L)a increased gradually up to a stable value with increasing gas phase flow rate; when the gas flow rate was relatively small, k(L)a decreased with the increase of the liquid flow rate; while the gas flow rate reached a critical flow rate, k(L)a increased with increasing liquid phase flow rate. Several dimensionless criterions were correlated to predict k(L)a, and the calculated values coincided well with experimental data. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.