The migration of potassium in an iron/H-ZSM-5 bifunctional system was investigated by pressing K/Fe2O3 and H-ZSM-5 in a pellet using 2 t of pressure. These pellets were heated at 350 degrees C in air for a number of days. Migration of potassium was visualized using energy-dispersive X-ray profiling. The distribution of potassium in the Fe2O3 phase and the H-ZSM-5 phase was approximately constant, with a step change over the phase boundary. The step change varied as a function of the heating time. The amount of potassium migrated from the Fe2O3 phase to the H-ZSM-5 phase was quantified using NH3-TPD. It is shown that an equilibrium distribution between potassium in the Fe2O3 phase and the H-ZSM-5 phase is obtained after ca. 7 days of heating.