The inner pores of ferrierite (FER) with a Si/Al ratio of 21 have been modified by plasma-polymerized propylene loading and subsequent thermal treatment under a flow of He at 550 degrees C in order to identify the exact role of carbonaceous deposits (coke) in the skeletal isomerization of 1-butene. It was found that even a small weight loading (0.8 wt%) of carbonaceous deposits within the inner pores of FER zeolite was sufficient to modify the pore structure of the zeolite, leading to a noticeable enhancement of the selectivity to isobutene. This may be attributed to the increased spatial constraints inside the pores which restrict undesired side reactions, such as 1-butene dimerization followed by cracking to light hydrocarbons.