The effect of calcination and hydrothermal treatments on the structure and properties of HZSM-5 zeolites with a range of aluminum contents has been investigated. Characterization of the treated zeolites was undertaken with solid-state NMR (Al-27 and Si-29), infrared, nitrogen and water adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, chemical analysis, and Xe-129 NMR spectroscopy, Both calcination and hydrothermal treatment were found to cause dealumination of the zeolite lattice and formation of extralattice aluminum species of low symmetry which remain within the pores of the zeolite. HZSM-5 with low aluminum content was found to be more resistant to dealumination by either method and the degree of dealumination was greater when steam was present in the treatment. Xe-129 NMR was found to be a useful probe for the presence of extralattice aluminum in the pores of lattice dealuminated HZSM-5.