The adsorption and reaction of 1-methylcyclopentene (MCPE) and 1-methylcyclopentanol (MCPOH) on HZSM-5 were studied by in-situ FTIR spectroscopy at 150-295 K. For MCPE, weakly adsorbed species, i.e., alkyl group adsorption on the acidic OH group, were predominant below 196 K. At 196 K and above, the pi -OH complex was formed followed by stepwise formation of stable alkenyl carbenium ions and alkoxy species, characterized respectively by an IR band at 1520 cm(-1) and by the shift of C-H stretching frequencies. For MCPOH, dehydration occurs at 268 K and MCPE is produced. The latter further forms alkenyl carbenium ions and alkoxy species progressively with the increase in temperature.