Visualization of mass transfer is a powerful tool to improve understanding of local phenomenon. The use of an oxygen-sensitive dye (colorimetric technique) (Dietrich et al., Chem Eng Sci. 2013; 100:172-182) has showed its relevancy for locally visualizing and characterizing gas-liquid mass transfer at different scales (Kherbeche et al., Chem Eng Sci. 2013; 100: 515-528; Yang et al., Chem Eng Sci. 2016; 143:364-368). At present, the occurrence of a possible enhancement of the gas-liquid mass transfer by this reaction has not been yet demonstrated. This article aims at filling this gap by evaluating the Hatta number Ha and the enhancement factor E associated with the oxygen colorimetric reaction when implementing in milli/micro channels. For that, as data on the kinetic of the colorimetric reaction are seldom in the literature, the reaction characteristic time was first estimated by carrying out experiments in a microchannel equipped with a micromixer. The diffusion coefficients of dihydroresorufin and O-2 were then determined by implementing two original optical methods in a specific coflow microchannel device, coupled with theoretical modelling. The knowledge of these parameters enabled at last to demonstrate that no enhancement of the gas-liquid mass transfer by this colorimetric reaction existed. Complementary information about the reliability of the colorimetric technique to characterize the gas-liquid mass transfer in milli/micro systems was also given. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 2272-2284, 2017