Specific degradation reactions of the ‘reactive isomer’ of the [C2B10H13]– anion, [r-C2B10H13]–, result in the removal of one or two carbon atoms from the cage to produce a series of monocarbaboranes 9-Me-10-HO-nido-[7-CB10H11]–, 8-Me3N-arachno-4-CB8H12, and arachno-4-CB7H12, together with the binary borane 2-Me-nido-B6H9; further reactions give the compounds 8-Me3N-9-Me-nido-6-CB9H10, arachno-4,6-CSB7H11, and closo-[1-CB7H8]–.