Previous in situ NMR studies of the reactions of propene and ethylene on zeolites have claimed the formation of small amounts of cyclopentenyl carbenium ions. The present investigation clearly demonstrates the formation of these ions in high yield from cyclic precursors on acidic zeolites. In situ C-13 solid-state NMR experiments are reported for the reactions of cyclopentene, cyclopentanol, cyclohexanol, and cyclohexane on a variety of zeolites of differing acid strength. The reaction sequence for the alcohols was dehydration, double-bond migration, oligomerization, C-6 to C-5 ring equilibration, and accumulation of cyclopentenyl ions. In some of the experiments up to one-third of the Cg ring units of the oligomers were positively charged. A C-13 chemical shift anisotropy measurement verified that the downfield signals are due to carbenium ions. Ex situ identification of the neutral extractable oligomers combined with comparison to model carbenium ion shifts permitted a plausible identification of the specific ions formed. The formation of products containing five-membered rings from cyclohexanol was modeled by the cyclohexane-methylcyclopentane skeletal rearrangement reaction. This investigation firmly establishes the formation of cyclopentenyl ions in zeolites and considers their significance in regard to reaction mechanisms. The persistence of these ions in high concentrations in zeolites is shown to be a consequence of the relative acid strength of the zeolite and the acid strength of the carbenium ions. This conclusion has important implications for the spectroscopic detection of reaction intermediates and the establishment of an acidity scale for zeolites.