Pentaborane(9), B5H9, and diborane(6), B2H6, undergo hydrogen isotope exchange with deuterated aromatic hydrocarbons. Lewis acid catalyzed hydrogen isotope exchange occurs between benzene-d6and the apical hydrogen atom of B5H9 to form 1-DB5H8 at ambient temperature. In uncatalyzed exchanges, B5H9 reacts with deuterated aromatic hydrocarbons to produce 1, 2, 3, 4, 5-D5B5H4at +45 °C and B5D9 at +120 °C. This thermally induced hydrogen isotope exchange apparently occurs via a reversible hydroboration of the aromatic ring. Diborane undergoes a similar isotope exchange with benzene-d6under mild thermal conditions.