Unimolecular and collision induced fragmentations of cis- and trans-1-acetyl-2-methylcyclopropanes, complexed to bare Fe+ cations, were examined by means of tandem mass spectrometry. Stereoselective isotopic labeling data as well as comparative studies with acylic isomers reveal that two reaction pathways are in competition with each other. On the one hand, dehydrogenation, loss of ethene, and loss of acetaldehyde proceed via a common intermediate which is formed via ring cleavage of 1-acetyl-2-methylcyclopropane/Fe+ to yield the corresponding 3-hexen-2-one/Fe+ complexes; these undergo subsequently CH- and CC-bond activation via the remote functionalization mechanism. These ring cleavages are associated with a complete loss of stereochemical features for the cis and trans isomers. In contrast, unimolecular decarbonylation occurs stereoselectively and is favored for the trans isomers; this is rationalized via a stereoselective formation of cis- and trans-eta(3)-allyl complexes in the course of the electrocyclic ring-opening process. Furthermore, interconversion reactions, thermodynamic and kinetic aspects, and isotope effects of the CH- and CC-bond activation processes are discussed.