Ethylbenzene reactions on HMOR and Pt–HMOR have been studied by means of solid-state MAS NMR. These experimental findings clearly indicate that disproportionation via the intermediacy of diphenylethane structure constitutes the first steps of this catalytic process over both HMOR and Pt–HMOR. In addition, ethylbenzene is suggested to undergo nonbranching rearrangements via the intermediacy of a two-electron–three-center bond leading to a 13 C-label transfer from α- to β-position of the side chain. This appears to be a major catalytic event over both catalysts. At later stages, disproportionation of ethylbenzene via dealkylation/alkylation steps accompanied by secondary reactions of ethylene proceeds freely on HMOR, whereas it is very much suppressed on Pt–HMOR. While dealkylation is very pronounced over Pt–HMOR, alkylation is very much limited by rapid hydrogenation of ethylene to ethane on Pt sites. Based on the spectroscopic evidence of this investigation, a number of schematic representations are illustrated to address the mechanistic approaches taken in this contribution.