[en] The isomerization and the D₂-exchange of 2,3-dimethyl-1 -butene has been studied on an n-alumina which had been pre treated at only 300 deg C. Isomerization and D₂-exchange reactions were both followed in a conventional microcatalytic pulse reactor, using helium or H₂ as a carrier gas for the isomerization experiments and D₂ for the exchange experiments. Reactant and product were separated by gas chromatography on a 2 m dimethylsulfolane/chromosorb-column at 25 deg C. The separated substances were then analyzed by NMR and mass spectroscopy. The catalysts were deuterated by exchange in D₂O vapour at 300 deg C. for isomerization experiments on deuterated surfaces. The results strongly suggest an intermolecular hydrogen transfer, if the initial activity had been reduced by self-poisoning. Initially existing sites for an intramolecular hydrogen transfer are rapidly eliminated by oxygen containing residues. These observations are discussed and a proton-donor-acceptor mechanism through a cyclic, carbanion-like intermediate bound to a suitable hydrogencontainig ''active site'' is proposed. (K.A.)