Gas–liquid mass transfer was investigated around Taylor bubbles flowing in a long, in-plane, spiral-shaped milli-reactor involving various configurations. Using a colorimetric technique and image post-treatment, the variation of the equivalent O2 concentration inside the liquid slugs was measured. The coloration positions, corresponding to 99% of the maximum concentration reached in the liquid slug, changed significantly under various operating conditions. Overall volumetric mass transfer coefficients were evaluated on the basis of these coloration positions, without considering the hydrodynamics change. The intensification factor of two configurations was found to be proportional to the increase in Dean number. Considering the changes in pressure drop, bubble length and velocity, the axial variation of cumulated mass flux could be measured. It decreased along tube and finally tended towards a constant value. A scaling law for the Sherwood number was proposed by introducing a normalized time, dimensionless liquid slug length, and the Péclet and Dean numbers.