The influence of the isomorphous substitution of Al (by B and Ga) and Si (by Ge) on the catalytic activity of zeolite systems is investigated using a range of reactivity indexes, some of them being density functional theory based. From the values of the local softness and the charge on the hydrogen atom of the bridging hydroxyl, used as a first approximation to the local hardness, it is concluded that the acidities of the zeolite-type model systems, used in the calculations, are dependent on several characteristics which are of importance within the framework of the hard and soft acids and bases (HSAB) principle. These include the hardness (softness) and electronegativity of the substituting atoms, properties which are closely related to the corresponding group values. The OH-bond length and the ionicity of the OH bond were found to be suitable descriptors of the acidity, whereas the dipole moment derivative with respect to the OH-bond length appears to fail as a reactivity index in this case.