Ab initio quantum chemical methods have been used to study the mechanisms for the hydrocarbon conversion in zeolites (cracking, isomerisation, alkylation, etc.). It is found that the intermediates are covalent alkoxide species and indicate that there is no energy ordering of these alkoxides according to their primary, secondary, or tertiary nature. Transition states have been located for the most important conversion reaction steps for hydrocarbons up to C6. The reactions are concerted and the transition states are ionic and ring-like. Due to charge delocalisation in the transition state the activation energies depend on the nature of the initial and final alkoxides. These calculations are the foundation for a new model of the hydrocarbon conversion which can better describe the influence of the zeolite as will be illustrated with calculations on acid sites with varying acid strength.