We present a new concept for studies of the kinetics of fast gas liquid reactions. The strategy relies on the microfluidic generation of highly monodisperse gas bubbles in the liquid reaction medium and subsequent analysis of time-dependent changes in bubble dimensions. Using reactions of CO2 with secondary amines as an exemplary system, we demonstrate that the method enables rapid determination of reaction rate constant and conversion, and comparison of various binding agents. The proposed approach addresses two challenges in studies of gas liquid reactions: a mass-transfer limitation and a poorly defined gas liquid interface. The proposed strategy offers new possibilities in studies of the fundamental aspects of rapid multiphase reactions, and can be combined with throughput optimization of reaction conditions.