Most authors publishing numerical calculations for the how field claim that scientific progress on liquid mixing in bubble columns must be based on the fluid dynamics. In a previous paper (Millies, M. and Mewes, D., 1995, Calculation of circulating flows in bubble columns. Chem. Engng Sci. 50, 2093-2106) a calculation method was presented, which allows the prediction of the flow field including several consecutive circulation cells. The conclusions for liquid mixing are drawn in the present paper. The main transport resistance of mixing of liquid phase is located between the circulation cells, a region which was completely neglected up to now. A structured model and even the one-dimensional dispersion model may be obtained from simplifying the transport equation of tracer under certain assumptions. Thus, the coefficients of the structured model and the one-dimensional dispersion model are obtained as results of the numerical calculation. Even though the stirred tank model appears convincing on first view, it is shown to fail as a link between the fluid dynamics and the liquid mixing.