Bicyclo[3,2,1]octa-2,6-diene (I) undergoes base-catalysed hydrogen-isotope exchange with dimethyl sulphoxide containing potassium t-butoxide exclusively at the 4-position and 10⁴ times more rapidly than comparable exchange of bicyclo[3,2,1]oct-2-ene or cyclohexene. Reprotonation of the intermediate anion always occurs to regenerate (I) so that tricyclo[3,2,1,0^2,7]cot-3-ene (IIa) and tetracyclo[3,3,0,0^4,6,0^2,8]octane (IVa) were not detected. Neither of these latter hydrocarbons undergoes isomerisation or deuteriation in the solvent–base system. The stereochemistry and detailed mechanism of exchange are discussed.