ZSM-5 zeolites with different Si/Al ratios (15, 25, 40) were impregnated with phosphorus (0.5-3 wt%) in form of H3PO4 or NH4H2PO4. The samples were characterized before and after severe hydrothermal treatment by XRD, IR, Al-27 Si-29, P-31 MAS NMR, and their activity for the cracking of n-decane was measured. It Was found that phosphorus impregnation increases the hydrothermal stability of framework aluminium whatever the source of phosphorus used. The acidity and cracking activity of steamed samples reached all optimum for a P/Al molar ratio of ca. 0.5-0.7. A chemical model for the phosphorus-zeolite interaction is proposed in which the framework aluminium pairs are stabilized by extra-framework cationic species formed by protonation of orthophosphoric acid. The influence of isolated versus pairs Of aluminium on activity and selectivity after steaming is discussed. When P-impregnated ZSM-5 sample is used as an additive for cracking industrial feeds, selectivity to propylene and butenes increases. (c) 2005 Elsevier Inc. All rights reserved.