Video image analysis techniques were used to investigate the solids mixing behavior in a two-dimensional (thin) fluidized bed containing "simulated" horizontal tube banks. Colored glass beads were used as the tracer, and the image intensity was used to deduce the variation of axial concentration profiles with time. A three-phase counter-current back-mixing model was developed for data interpretation. The model accounted for the axial variations in bubble properties and in flow (of gas and solids) in the presence of tubes. These variations induce cross-flow of material between phases (for volume conservation). Mathematical form for the cross-flow term for inclusion in the conservation equations was deduced. The model equations were solved using finite difference techniques; bubble parameters for use in the model have been described in a previous paper, part 1 in this series. Calculations compare favorably with experimental data. The results also show that both tube position and tracer feed location have significant effects on the mixing time. Tubes located in the top and the bottom of the bed lead to smaller mixing times; tracer fed in the middle of the bed provides the fastest mixing rate.