The dynamic response of a gas-solid fluidized bed to a pulse input of the tracer at the top of the bed is obtained. The tracer distribution between the phases is represented by a convective model. The model includes a fast downflow of dense phase at the wall parallel with the tubes. But for this, the model is very similar to the countercurrent backmixing model. The simulation is performed by using a numerical model (“cinematic” model), and the results are in good agreement with available results.