Ab initio molecular-orbital calculations using the 3-21G basis set are performed to study the protonation reaction of propylene and isobutene by zeolite bridged hydroxyls ZOH, which are simulated by a cluster model which consists of two Si tetrahedra and one Al tetrahedron. We have extended this study to include clusters with different compositions by introducing B and Ga in T-III positions. The calculations show that in all reactions the adsorption of the olefin molecule on;the acidic OH group takes place, leading to a stable pi-complex with a structure similar to those of the isolated olefin and clusters constituents. The pi-complex is transformed into a zeolite-alkoxide of covalent characteristics. The surface alkoxides are the most stable structures with reaction energies in the range of -15 to -17 kcal/mol. These reactions take place through transition states whose organic fragment has a geometry and charge distribution resembling those of the 2-propyl and tert-butyl classical carbenium ions for zeolite-propylene and -isobutene, respectively. The obtained activation energies (30-40 kcal/mol) are of the same order of magnitude in all the reactions considered. The bifunctional mechanism of the reactions is rather complicated and implies a concerted process involving the proton transfer from the zeolite toward a carbon of the olefin double bond, and the simultaneous C-O, bond formation at the adjacent oxygen on the zeolite structure. The reaction mechanism and the properties of the transition states are practically the same, irrespective of the framework T-III atom and the increase in propylene-isobutene branching. It is then proposed that the geometric conformation of transition state can be more determinant than the chemical composition of the zeolite. In this respect, the flexibility of the zeolite structure plays an important role.