The Electronegativity Equalization Method (EEM) is used to study the dependence of the basicity of faujasitetype zeolites on the Si:Al ratio, the cation type, and the type of oxygen. The model structures were obtained by distance least-squares optimization within the F222 space group, and cations were located with the Monte Carlo technique, starting from experimental XRD positions. The main effect determining basicity, as indicated by the average negative charge on the oxygens, is the chemical composition of the framework. The average negative oxygen charge increases with decreasing Si:Al ratio, in agreement with the experimentally found increase of basicity. The secondary structure effect identifies the supercage oxygens O1 and O4 as the most basic. The details of the cation distribution further controls the basicity of these oxygens, which increases when Na + is replaced by Cs + on sites II and, especially, site III, also in agreement with experiment. Besides charges on oxygen, Fukui indices and regional sensitivities are also good indicators of basicity.Brønsted acid sites in zeolites, the so-called bridging hydroxyls tSisOHsAlt, have been extensively studied, both experimentally and theoretically. 1-8 Its acidity is chemical composition and structure dependent, the former being the most important and increases with increasing Si:Al ratio. As soon as the Si:Al ratio has reached the level that, statistically, no next nearest neighboring Al atoms (NNN) are present, the maximum acidity is reached. 4-7 This was theoretically addressed recently using the electronegativity equalization method (EEM). 8 Following a classical acid-base pair concept the surface oxygen is the conjugated base of the acidic bridging hydroxyl. The basicity of these oxygens is called the structural or intrinsic framework basicity. 9-11 As for the bridging hydroxyl we expect the properties of the basic sites to depend on the chemical composition (Si:Al ratio and the exchangeable cations) as well as on the structure type.Up to now, the intrinsic basicity of zeolites has been studied experimentally, either directly, by looking at the properties of the framework atoms, [9][10][11][12] or indirectly, by adsorption of probe molecules. [13][14][15] The O(1s) binding energy, deduced from XPS spectra, reflects the electron density of the oxygen atom in the framework. It decreases with increasing Al content, but at a given Si:Al ratio it increases with increasing electronegativity of the cation. 16,17 A decrease of the O(1s) binding energy is equivalent to an increase of the electron density around the O nucleus, or an increase of the negative charge on the oxygen, thus reflecting an increased basicity. XPS, however, gives only general trends and no distinction can be made between structurally different atoms. Similar results are obtained from calculations of averaged charges on framework oxygens with Sanderson's model of electronegativity: 18 the average negative charge on structural oxygens increases with the Al content of the framework and with dec...