Liquid mixing in the riser and downcomer of two- and three-phase internal loop airlift reactors as well as the overall liquid mixing were studied. Tracer was injected into the airlift reactors, and pH sensors were employed to measure the variations of tracer concentration in the riser, downcomer, top, and bottom sections. The Bo number in individual sections of the airlift reactors were obtained by time domain analysis of these concentration variations. It was found that the degree of mixing in the riser was higher than that in the downcomer. Moreover, mixing in the two-phase system was superior to that in the three-phase system, and the degree of mixing of the calcium alginate system was lower than that of the polystyrene system. Minimum values of the overall axial dispersion coefficient existed in the relationship between the overall axial dispersion coefficient and solids loading.