H-ZSM-5 samples were pretreated with dilute NaOH and then transformed into the acid state. The variation of framework Al was studied by Al-27-MAS-NMR spectroscopy, the Al gradient by XPS and the OH groups by IR spectroscopy. The catalytic activity was studied in the conversion of propane and of methanol. The treatment of the parent zeolite (5.2 Al/u.c.) resulted in a slight decrease of the content of framework Al but in a strong enrichment of nonframework Al on the outer surface. The catalytic effect was an increased aromatization of propane and a prolonged lifetime in the methanol conversion. The treatment of a dealuminated sample, containing much nonframework Al, caused a partial reinsertion of Al into the framework. This modification leads to an enhanced aromatization of methanol. Thus treatment with NaOH can control the content of framework Al and the catalytic activity, selectivity, and stability.