The unimolecular decompositions of 5-methylheptanenitrile (1), 6-methyloctanenitrile (2), and 7-methylnonanenitrile (3) complexed to bare Co+ cations have been investigated by means of tandem mass spectrometry. Extensive regio- and diastereospecific labeling experiments were conducted to gain further insight into the mechanistic details, in particular the stereoselectivity of the Co+-induced demethanation in the gas phase. The overall reaction follows a 1,2-elimination mode, and, with regard to the regiospecificity, the neutral methane originates exclusively from the (omega-2) and (omega-1) positions of the alkyl chain. For 2 and 3 the examination of diastereospecifically labeled isotopomers reveals that the gas-phase demethanation exhibits a remarkable diastereoselectivity. This unprecedented example for a stereoselective C-C bond activation in gas-phase oganometallic chemistry can be accounted for in terms of steric features of the metallacyclic intermediates.