The effect of the introduction of phosphorus on H-MFI zeolite before and after hydrothermal treatment was studied. The samples were characterized by XRF, nitrogen adsorption, Al-27 and P-31 MAS NMR, n-propylamine temperature-programmed decomposition, n-hexane cracking and vacuum gasoil cracking. Results showed that P stabilizes the MFI zeolite structure by preventing the segregation of the framework Al atoms during the steaming treatment. Although there is an optimum phosphorus content, its effect on the selectivity towards light olefins will depend on the severity of the steaming treatment, i.e., with more severe treatment the stabilization is more evident. NMR results evidenced the presence of several types of aluminium and phosphorus species. Besides the classical framework tetracoordinated Al, which is the main responsible for the acidity of the samples, another tetrahedral aluminium probably in a distorted environment also seems to be active in n-hexane cracking, besides favouring the light olefins production during gasoil cracking. (c) 2006 Elsevier B.V. All rights reserved.