Gas-liquid reactions in microreactors play an important role in scientific research and industry. Enhancing heat and mass transfer allows overcoming mass transfer limitations in gas-liquid reactions. Mass transfer can be further increased by employing helically coiled capillaries, which induce Dean vortices and improve radial mixing. A colorimetric technique is proposed for gas-liquid reactions in straight and helically coiled capillaries in order to visualize local mass transfer phenomena and concentration distributions. This method is based on the consecutive oxidation of leuco-indigo carmine and enables noninvasive investigation of mass transfer and chemical selectivity in microchannels with high temporal and spatial resolution.