The present paper focuses on the local characterization of gas-liquid mass transfer in a straight millimetric square channel, as constituting the preliminary step required for performing gas-liquid reactions in such devices. For this purpose, a new colourimetric technique using an oxygen sensitive dye was developed. It was based on the reduction of a colourimetric indicator in presence of oxygen, this reduction being catalysed by sodium hydroxide and glucose. In this study, resazurin was selected as the colourimetric indicator as it offered various reduced forms, the colours of which ranged from colourless (without oxygen) to pink (when oxygen was present). Thus the mass transfer around bubbles flowing in a straight millimetric square channel could be visualized in space and time. Some pictures were recorded by a monochromatic CCD high speed camera and, after post-processing, the shape, size and velocity of the bubbles, and the grey-level maps around them were measured. A calculation method was also developed to determine the transferred oxygen fluxes around the bubbles and the associated liquid-side mass transfer coefficients. The results compared satisfactorily with global measurements made using oxygen microsensors (Roudet et al., 2011. Hydrodynamics and mass transfer in inertial gas-liquid flow regimes through straigth and meandering millimetric square channels. Chem. Eng, Sci, 66, 2974-2990). This study constitutes a striking example of how interesting a tool this new colourimetric method could be for investigating gas-liquid mass transfer in transparent fluids with a view to quick millireactor design. (C) 2013 Elsevier Ltd. All rights reserved.