The skeletal isomerization of 1-pentene at 1.2 bar total pressure and 30 mbar 1-pentene partial pressure was investigated by in situ IR measurements over FER and BEA zeolites. If the reaction was carried out isothermally small loss of Bronsted acid sites due to coke formation, constant conversion and high selectivity to isopentenes was found over FER. A loss of acid sites and decrease in conversion was found over BEA, but the selectivity to isomerization increased remarkably. The pore size of FER favored the monomolecular reaction of 1-pentene and suppressed bimolecular reactions. In the bigger pores of BEA initially dimerization cracking and hydrogen transfer reactions leading to coke layer took place then the skeletal isomerization of 1-pentene prevailed.