Several methods were used to evaluate the potential role of bimolecular mechanism in skeletal isomerisation of n-butene to isobutene, Dimers of n-butene were prepared and their reaction monitored. Product distributions of these reactions were compared with those obtained with n-butene. Comparison was performed with different catalysts at 1 bar total pressure in an apparatus which makes both continuous dow and pulse experiments possible. Com positions of products of the reactions of n-butene and of dimers were found to be very different an catalysts selective to isobutene, On highly acidic catalysts, which form isobutene nonselectively and show a high formation of by-products (C-3(=) and C-5(=)), distributions were similac. Skeletal isomerisation is suppressed by a higher pressure of butene while the rate of formation of by-products increases with increasing pressure. Optimum temperatures of dimerisation and of skeletal isomerisation differ considerably. All these results indicate that bimolecular mechanism is responsible mainly for the byproducts. It is concluded that the skeletal isomerisation of n-butene to isobutene runs via a monomolecular mechanism whereas the by-products are formed by cracking of dimers of n-butene. Another source of the by-products seems to be dimers arising from isobutene, the latter being first formed by a monomolecular skeletal isomerisation of n-butene. The formation of by-products is suppressed when 10-membered ring zeolite is used as a catalyst.