Several known volatile closo-carboranes (1,5-C₂B₃H₅, 1,6-C₂B₄H₆, 2,4-C₂B₅H₇, 1,10-C₂B₈H₁₀) and their alkyl derivatives and ten new alkyl derivatives of these carboranes were identified from the thermal gas-phase flash reactions of B₂H₆, B₃H₇CO and B₄H₁₀ with ethyne, propyne and but-2-yne. In addition, nido-C₄B₂H₆ and its methyl derivatives were obtained from B₂H₆-alkyne flash reactions. All carboranes found in these high-energy reactions are the most thermally stable isomers of their class. The non-isolable borane {B₃H₇} is suggested as the initiator in all thermal flash reactions reported here.