Liquid-liquid mass transfer mechanism with slug flow in microreactor is investigated by means of experiments in square microchannels of 0.2 and 0.3 mm width. An experimental bench has been designed and an analytical protocol developed in order to follow the transfer of a solute from the dispersed phase to the continuous phase. The methodology used allows the identification of droplet side mass transfer coefficients. Total flow rate ranges from 5 to 50 mL.h-1, resulting in droplets velocities of 0.02 to 0.35 m.s-1. Volumetric mass transfer coefficient kda values obtained in the present work ranges from 0.89 to 9.45 s-1. The results reasonably fit with a model suggested by a previous study based on 2D direct numerical simulations. A comparison with other models available in literature shows that in the operating conditions considered in this work, the flow pattern inside the confined droplets at microscale leads to an enhancement of mass transfer compared to droplets that are not confined.