The role of carbonaceous deposits in the skeletal isomerisation of n-butene was studied in a pulse reactor. High activity and selectivity were observed under conditions when the catalyst was almost free of carbonaceous deposits. Carbonaceous deposits deactivate the catalyst and suppress both isobutene and by-product formation. However, since reactions leading to byproducts are suppressed more strongly, the relative concentration (yield) of isobutene might increase during the first few pulses. it is concluded that a selective monomolecular mechanism operates on the uncovered OH-groups. Anchoring hydrocarbon molecules on a surface is a more difficult step than subsequent oligomerisation. Deposits (mostly oligomers) that form early, crack non-selectively to isobutene and larger amounts of propene and pentenes.