The gas-phase reaction between 1-MeB 5 H 8 and propyne at 230 o C yields methyl derivatives of nido-2,3-dicarbahexaboranes and closo-2,4-dicarbaheptaboranes. The reaction between 1-MeB 5 H 8 and 2-pentyne at 190 o C gives analogous alkyl nido-carbaboranes (but not closo-carbaboranes). Detailed boron and proton NMR data on these products in conjunction with ab initio/GIAO/NMR computations show that the dicarbaboranes 2,4-Me 2 -2,3-C 2 B 4 H 6 (3) and 2-Et-3,4-Me 2 -2,3-C 2 B 4 H 5 (12) are produced in the gas-phase reactions of B 4 H 1 0 with ethyne and propyne, respectively. Early work on these two compounds identified them incorrectly as the first tricarbaboranes 2,3-Me 2 -2,3,4-C 3 B 3 H 5 and 2-Et-3,4-Me 2 -2,3,4-C 3 B 3 H 4 , respectively.