Hydroboration of Et2BC.tplbond.CBEt2 with tetraethyldiborane(6) leads to a B-ethylated tetracarba-nido-octaborane (I), a spiro-carborane (II), which belongs to the 2,3,5-tricarba-nido-carborane family, and a hexacarba-arachno-dodecaborane(12), along with polymeric material. The x-ray structure anal. of I, detd. here, is fully consistent with the structure deduced earlier from NMR. The structure of II in soln. was established by theor. anal. of its NMR data. Ab initio calcns. on the structures on B-methylated analogs of I and II and the comparison of calcd. with exptl. NMR support the suggested structures in soln. The calcd. structure of the carborane cage of I also agrees with the exptl. geometry.