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Density functional theory has been used to determine the effects of local composition and structure on the proton affinity (PA) of H-ZSM-5. These calculations were performed with clusters ranging in size from 34 to 41 atoms. For isolated Al sites, the associated PA is 329 kcal/mol. Introduction of a defect in the form of a silanol group immediately adjacent to such a Brønsted acid site reduces the PA to 316 kcal/mol. For an isolated structure containing two Al atoms in next-nearest-neighbor T sites, the value of PA lies between 330 and 298 kcal/mol, depending on the location of the two charge-compensating cations, and is not affected by the presence of a silanol group on the Si atom situated between the two Al atoms. However, the presence of a trigonally coordinated Si atom immediately adjacent to the Brønsted acid site reduces the PA dramatically to 281 kcal/mol, but the presence of trigonally coordinated Al in the vicinity of a Brønsted acid site has no effect on the value of PA. However, the presence of an extraframework Lewis acid center in the form of AldO + will lower the PA for a proton associated with one of the Al atoms in a structure containing two next-nearest-neighbor Al atoms, to 305 kcal/mol. Defects situated on Al atoms have PAs lying between 338 and 320 kcal/mol.