The　adsorption　and　transformation　of　cyclopentadiene　on　HY　and　HZSM-5　zeolites　were　investigated　by　infrared　(IR)　spectroscopy　and　temperature-programmed　desorption　(TPD).　The　stoichiometric　formation　of　monomeric　cyclopentenyl　carbenium　ions　(C5H+7)　was　observed　on　the　acidic　sites　in　the　supercages　of　zeolite　HY　and　in　the　channels　of　zeolite　HZSM-5　at　room　temperature,　without　formation　of　oligomerized　cyclopentadiene.　The　IR　spectra　indicate　that　addition　of　quantitative　cyclopentadiene　led　to　the　stoichiometric　consumption　of　acidic　OH　groups.　These　cyclopentenyl　carbocations　formed　in　the　supercages　affected　the　vibration　of　the　remaining　OH　groups　at　both　high　and　low　frequencies,　resulting　in　a　shift　of　the　OHHF　from　3642　to　3530　cm-1　as　well　as　a　shift　of　the　OHLF　from　3552　to　3500　cm-1.　The　TPD-MS　results　reveal　that　the　cyclopentadiene　transformation　on　these　H-form　zeolites　occurred　at　a　temperature　range　of　473-800　K　and　followed　a　hydride　ion-transfer　pathway.　The　monomeric　cyclopentenyl　carbocation　was　the　key　intermediate　initiating　the　cracking　chain　proceeding　by　the　cationic　mechanism.　©　2008　Elsevier　Inc.　All　rights　reserved.