The ring-opening reactions of methyloxirane and ethyloxirane on ZSM-5 catalysts are dramatically different in spite of otherwise identical reaction conditions, Even though both reactants undergo single ring opening providing isomers of the starting compound, the major transformation route of methyloxirane is dimerization resulting in dioxolane and dioxane derivatives of various kinds. This reaction pathway, however, is completely lacking in the case of ethyloxirane. Molecular modeling revealed that possible dimers of ethyloxirane are too bulky to be formed and diffuse through the channel system of the ZSM-5 structure.