Starting with HOOC-CB11Me11- or CB11Me12- as the starting material, collision-induced dissociation has produced a series of methylated analogs of didehydro-closo-carbadodecaborane anions by sequential losses of up to 5 equiv of ethylene. These reactive intermediates are carborane analogues of doubly pyramidalized alkenes and, more distantly, arynes. Density functional theory calculations have been used to develop a proposal for the mechanism of the unusual formation of ethylene from the carborane methyl substituents.