The catalytic conversion of methanol to olefins on zeolites is an industrially important process, yet the mechanistic details remain unresolved. Enylic cations (unsaturated carbenium ions) are active intermediates in the production of olefins and aromatics. However, these long-lived species are also precursors to carbonaceous deposits, the accumulation of which is responsible for catalyst deactivation. The aim of this work is to develop a mechanistic understanding that will ultimately allow steering the surface chemistry toward active intermediates. An in situ spectroscopic approach is applied to determine the nature of the relevant surface species and to track their transformations. The UV–vis and IR spectra of enylic cations dissolved in liquid acid and sorbed on zeolites are found to exhibit near identical characteristic absorption bands and are used to provide details about the species present during catalysis. The controversial IR bands appearing near 1510 and 1490 cm−1 during reactions of hydrocarbons and alcohols on H-zeolites are assigned to the allylic stretching vibrations of alkylcyclopentenyl cations with either a hydrogen or methyl group at the center of the allylic system. Furthermore, the zeolite framework is found to determine the type of alkylcyclopentenyl cations generated. Spectroscopic signatures indicate the presence of both polymethylbenzenium ions and polyalkylcyclopentenyl cations during the conversion of methanol on zeolite H-ZSM-5. To elucidate their role, alkylcyclopentenyl cations are synthesized in the absence of methylbenzenium ions from 2,6-dimethyl-2,4,6-octatriene on H-ZSM-5. These alkylcyclopentenyl cations are shown to be intermediates to gas-phase olefins and aromatics by in situ temperature-programmed reaction spectroscopy coupled with online gas-phase product analysis. The UV–vis band associated with alkylcyclopentenyl cations blue shifts during the reaction indicating a reduction in the number of carbons that constitute the cation, which matches the size of the concomitantly produced olefins. Additionally, the concentration of Brønsted acid sites is shown to control the hydride transfer pathway to isobutane and alkylcyclopentenyl cations. Alkylcyclopentenyl cations are revealed to be ubiquitous to numerous reactions of alcohols and hydrocarbons in microporous acid catalysts. These spectroscopic findings and mechanistic details provide fundamental insights that are of paramount significance to the development of catalysts that can effectively control selectivity.