The new tris(perfluoroalkyl)borane carbonyls, (C(2)F(5))(3)BCO and (C(3)F(7))(3)BCO, were prepared by means of a novel synthetic route using commercially available precursors by reacting K[(C(2)F(5))(3)BCOOH] and K[(C(3)F(7))(3)BCOOH] with concentrated sulfuric acid in the last step. The carboxylic acids, K[(C(2)F(5))(3)BCOOH] and K[(C(3)F(7))(3)BCOOH], were prepared by oxidative cleavage of the C[triple bond]C triple bonds in Cs[(C(2)F(5))(3)BC[triple bond]CPh] and Cs[(C(3)F(7))(3)BC[triple bond]CPh] in a two-step process to yield K[(C(2)F(5))(3)BCO-COPh] and K[(C(3)F(7))(3)BCO-COPh] as isolable intermediates. Crystal structures were obtained of K[(C(2)F(5))(3)BCO-COPh], K[(C(2)F(5))(3)BCOOH].H(2)O, (C(2)F(5))(3)BCO, K[(C(3)F(7))(3)BCOOH].2H(2)O, and (C(3)F(7))(3)BCO. In the crystal structures of (C(2)F(5))(3)BCO and (C(3)F(7))(3)BCO the C[triple bond]O bond lengths are 1.109(2) and 1.103(5) A, respectively, which are among the shortest observed to date. Tris(pentafluoroethyl)borane carbonyl and (C(3)F(7))(3)BCO slowly decompose at room temperature to yield CO, difluoroperfluoroalkylboranes and perfluoroalkenes. The decomposition of (C(2)F(5))(3)BCO was found to follow a first-order rate law with E(a)=107 kJ mol(-1).