The catalytic activities of the two catalysts ZSM-22 and ZSM-35 were compared in the skeletal isomerization of 1-butene. ZSM-22 demonstrated higher activity in 1-butene transformation, compared to that of ZSM-35. ZSM-35 was not selective to isobutene until non-selective acid sites had been poisoned by coke deposits, while ZSM-22 was selective already from the beginning. Information about the extent of coke formation was obtained by FTIR experiments and surface area measurements. ZSM-22 was more resistant towards coke formation, compared to that of ZSM-35. In order to obtain information about the reaction mechanism, 2,4,4-trimethyl-2-pentene and 1-octene were cracked over the catalysts. The selective mechanism for isobutene formation in the skeletal isomerization of 1-butene was most likely monomolecular. The bimolecular mechanism is not selective to isobutene, although it can contribute to the overall isobutene production.