Gas / liquid contact apparatuses are widely used in chemical, biochemical or pharmaceutical industry to provide and transfer gas species as reactant from the gaseous phase to pre-dissolved reactants in the liquid phase enabling a preferred reaction. The global and local transport are complex interlinked processes and therefore in practice in reactor design industry, mostly empirically correlated. For a secure control of the overall process and a more efficient reactor design, the local transport processes at gas / liquid interfaces need to be investigated in complexity reduced systems to be understood. Elongated bubbles, Taylor bubbles, in vertical channels 5.5 < D < 8 mm overcome the problem of dynamic shape deformation, complex 3D rise trajectories and they have a volume independent rise velocity, which make them the ideal experiment for reliable and reproducible investigations. Detailed optical measurements of global and local processes via high-speed Shadowgraphy, 2D2C PIV and p-2D LIF give new insights into the dependency of local bubble shape and global terminal rise velocity, establish a Sherwood correlation of shrinking CO2 bubbles in various channel sizes and shows the coupling of local transport phenomena at the bubble interface and the mixing in the wake region.