Ion-exchange of HZSM-5 with dilute KCl (or NaCl) solutions leads to partially exchanged zeolites, KHZSM-5. The degree of exchange was also found to be a function of the type of cation used. With increasing aluminium content of the zeolites, 0.70 to 1.60 mass-%, the K/Al molar ratio decreased from 0.68 to 0.38. The catalytic properties of the samples were compared using n-hexane cracking as the test reaction. For the HZSM-5 zeolites and under the appropriate reaction conditions, the percentage conversion was proportional to the aluminium content. The KHZSM-5 samples, on the other hand, exhibited an almost complete loss of catalytic activity. The effective degree of poisoning as evaluated in terms of the test reaction is therefore considerably higher than that determined from the K/Al molar ratio. The catalytic and ion-exchange results are interpreted as indicating that, depending on the aluminium content, a potassium cation is influencing between one and three acid sites, thereby implying that the aluminium tetrahedra in ZSM-5 are not in an isolated state.