This present work aims at studying the gas-liquid mass transfer around a Taylor bubble during the bubble formation stage and just after. For this purpose, the resazurin-based colorimetric technique proposed by Dietrich et al. (2013) was applied in a square flow-focusing milli-channel. An original MATLAB program was developed to extract first the features of two-phase flow pattern, such as: the bubble length and velocity, the liquid slug length, and secondly the equivalent oxygen concentration fields inside the liquid phase by means of an ad hoc calibration procedure.. This study showed that, the equivalent O2 concentration fields in the liquid slugs right after the bubble formation presents complex patterns (three-dimensional and transitory). At a given liquid flow rate, an increase of gas-liquid flow rate ratio led to higher levels of accumulated equivalent O2 concentration fields in the liquid slugs. At a given gas-liquid flow rate ratio, decreasing liquid flow rates allowed to a rise of the accumulated equivalent O2 concentration fields in the liquid slugs. At last, the contribution of the bubble formation to the overall mass transfer was found significant.