The objective of this work is to study the hydrodynamics of the liquid phase and its interaction with the gas-phase hydrodynamics. The local liquid flow is investigated by means of thermal pulse anemometry. That is, local residence time distributions and local velocity distributions are experimentally measured. The study is carried out in the liquid upflow region and through the homogenous and heterogeneous flow regimes. Liquid mixing is analysed separately on the basis of two different assumptions: as a dispersive mechanism or as a convective mechanism. Moreover, the liquid velocity distributions are compared with global absolute bubble velocity distributions.