Ab initioquantum chemical methods have been used to study themechanisms for the hydrocarbon conversion in zeolites (cracking,isomerisation, alkylation, etc.). It is found that theintermediates are covalent alkoxide species and indicate thatthere is no energy ordering of these alkoxides according totheir primary, secondary, or tertiary nature. Transition stateshave been located for the most important conversion reactionsteps for hydrocarbons up to C6. The reactions are concerted andthe transition states are ionic and ring-like. Due to chargedelocalisation in the transition state the activation energiesdepend on the nature of the initial and final alkoxides. Thesecalculations are the foundation for a new model of thehydrocarbon conversion which can better describe the influenceof the zeolite as will be illustrated with calculations on acidsites with varying acid strength.