Skeletal isomerization of n-butenes to isobutene at 620 K was followed over NaH-ferrierites (FER) with different concentration of Broensted sites, prepared by NH4+ vs. Na+ ion exchange, and over H-FER containing both, the Broensted and Lewis sites formed by zeolite dehydroxylation at temperatures ranging from 670 to 990 K. Quantitative analysis of IR spectra of OH groups and adsorbed d(3)-acetonitrile on ferrierite enabled determination of the concentration of Broensted and Lewis sites, and detected also those present in 8-ring ferrierite channels. With H-FER, the yield of isobutene is linearly proportional to the concentration of acidic OH groups. Formation of C-3 and C-5 olefins, C-2-C-5 paraffins and higher molecular weight products, resulting from di-, oligomerization of butenes followed by cracking/hydrogen transfer reactions, is enhanced by the presence of Lewis sites, if simultaneously present with the Broensted sites, that is in a substantial change of the selectivity of the reaction. On the other hand, both the conversion and yield of isobutene are nearly constant at decreasing concentrations of OH groups in NaH-FER, and only at law-concentration range of acidic hydroxyls a decrease in conversion was found. This indicates that the exchanged Na+ ions are placed preferably in small ferrierite channels, as also evidenced by Na-23 MAS MMR, which size is not large enough to accommodate isobutene molecules. Thus, besides the well-known positive effect of the inner geometry of ferrierite channels on isobutene selectivity, it has been shown that the reaction of n-butenes to isobutene is proportional to the number of hydroxyls located in large channels of ferrierite, and simultaneous presence of Lewis sites enhances di-,oligomerization/cracking reactions leading to higher yields of by-products.