[en] Ion cyclotron resonance techniques are employed to determine the gas-phase Bronsted and Lewis acidities as well as the Bronsted basicity of 1-methyl-1,4-dihydroborabenzene, CH₃BC₅H₆. The ring proton is found to be highly acidic with PA(CH₃BC₅H₅^-) = 337 +- 3 kcal/mol. This acidity results from the formation of 6π electron aromatic anion CH₃BC₅H₅^-, which is isoelectronic with toluene. Both the Lewis acidity toward F^- as a reference base and the proton basicity of the parent molecule suggest that there is little interaction between the diene π system and the electron-deficient boron. This is further confirmed by the similarity of both negative and positive ion chemistry of the borabenzene to that of aliphatic boranes