We have investigated theoretically the interaction of cis-2-butene with a Bronsted acid site (BAS) embedded within a ring of 10 tetrahedral zeolite cluster (T10-OH). Calculations were performed at ab Initio SCF-MO level with the STO-3G basis set. Cs symmetry restriction was imposed for the optimization geometry of the T10-CH cluster, and no symmetry restriction was assumed for the cis-2-butene-HO-T10 interaction. This interaction first leads to the formation of a molecular complex, where the C=C is weakly coordinated to the proton of the BAS. Then, a hexa-cyclic transition state involving the coordination of one C atom of the C=C to one oxygen atom next neighbor of the OH group and the other C atom to the acid proton. Finally, an intermediate secondary alkoxy complex is formed. The alkoxide structure is a very stable complex, dominated by a covalent C-O bonding between the C atom and zeolite oxygen atoms. (C) 2002 Elsevier Science B.V. All rights reserved.