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, ²⁷Al, ²⁹Si, ³¹P 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 aluminum whatever the source of phosphorus used. The acidity and cracking activity of steamed samples reached an 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 aluminum pairs are stabilized by extra-framework cationic species formed by protonation of orthophosphoric acid. The influence of isolated versus pairs of aluminum 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.