The caesium and tetraethylammonium salts of the 1-cyanocarba-closo-dodecaborate anions [1-NC-closo-1-CB₁₁X₁₁]⁻ (X = H, F, Cl, Br, I) were synthesized from CsLi[closo-1-CB₁₁X₁₁] and phenylcyanate. All anions are stable against aqueous acids and bases, except for the fluorinated anion [1-NC-closo-1-CB₁₁F₁₁]⁻ that immediately reacts with water. The Cs⁺ and [Et₄N]⁺ salts of the anions were characterized by IR, Raman and multinuclear NMR spectroscopy as well as by MALDI and ESI mass spectrometry. The thermal properties of the Cs[1-NC-closo-1-CB₁₁X₁₁] (X = H, Cl, Br, I) and of some of their tetraethylammonium salts were studied by differential scanning calorimetry. The crystal structures of [Et₄N][1-NC-closo-1-CB₁₁H₁₁] and Cs[1-NC-closo-1-CB₁₁Cl₁₁] were determined and the discussion of the structural and spectroscopic properties is supported by density functional and (RI)-MP2 calculations. A first insight into the hydrolysis of the [1-NC-closo-1-CB₁₁F₁₁]⁻ anion is presented and its reactions with water are compared to those of other undecafluorinated carba-closo-dodecaborate anions. The reduced hydrolytic stability of the [1-NC-closo-1-CB₁₁F₁₁]⁻ anion in comparison to its homologues is related to differences in the carbon–boron and boron–boron bond lengths of the {closo-1-CB₁₁} cage and to different partial charges of the cluster atoms.