The adsorption properties and possible rearrangements of several proposed oxonium ylides and oxonium ions in protonated ZSM-5 are studied using the 2-layered ONIOM(B3LYP/6-31+g(d,.p):HF/3-21g) approach. We show that both methyl oxonium methylide and dimethyl oxonium methylide are highly energetic species and unlikely to be intermediates in the formation of a carbon-carbon bond as the zeolite lattice does not offer supplementary stabilisation. The trimethyl oxoniurn and ethyldimethyl oxonium ions, however, are distinctly stabilised by the surrounding ZSM-5 framework, which does not impose steric constraints on further intermolecular reactions. (c) 2005 Elsevier B.V. All rights reserved.