CO2 capture is still a major challenge for the environment and decarbonization of gas streams. It has been industrially feasible for several decades using chemical absorption by alkanolamine solvents. The purpose of this work is to characterize the mass transfer in a spray contactor during CO2 absorption using monoethanolamine as a solvent. A new, effective way to investigate mass transfer is to measure the refractive index during the reactive absorption. Indeed, the concentration and temperature evolution inside the droplet during absorption induces a modification of the refractive index. By using rainbow refractometry technique (GRT), the liquid side mass transfer coefficient was measured in a controlled gas atmosphere during a transfer process in aqueous 30% (w/w) monoethanolamine and compared to the literature.