The protonation of ethene, propene, 1-butene, and iso-butene by acidic theta-1 zeolite to give covalent alkoxide intermediates has been theoretically investigated by means of cluster and periodic calculations. The influence of the methodology used, of the olefin size and nature, of the cluster used to model the catalyst, of the degree of geometry relaxation around the active site, and of the long-range effects caused by the Madelung potential of the crystal has been analyzed. It has been found that the stability of alkoxide intermediates is very sensitive to the local geometry of the active site and to the nature of the carbon atom involved in the C-O bond, whereas the activation energy for the process is mainly influenced by the methodology used and by electrostatic effects, due to the carbocationic nature of the transition state. Finally, the question of whether the tert-butyl carbenium ion is a reaction intermediate or just a transition state in the iso-butene protonation process has been adressed by calculating the complete reaction mechanism at different theoretical levels.