Selective isomerization of butene to iso-butene has been studied with pure and variously modified alumina as catalysts. Results can be self-consistently explained by concluding that (i) the monomolecular mechanism operates and requires Lewis acid sites, the presence and quality of which can be modulated by heat treatment in hydrogen, by surface modification with halogens, and by application of certain catalytic poisons, and (ii) the bimolecular mechanism is related to the Bronsted acid sites and is stimulated, as expected, by increased pressure of n-butene.