Unimolecular fragmentations of aliphatic carboxylic acids complexed to bare Fe+ cations were examined by means of tandem mass spectrometry. Most of the reactions can be explained in terms of three basic mechanisms : (i) dehydration of the carboxyl group involving an cu-hydrogen atom of the acid, (ii) decarbonylation of the functional group, most likely via a carbene intermediate, and (iii) dehydrogenation and alkene losses from nonactivated CH- and CC-bonds ("remote functionalization"). Labeling studies establish that the latter reaction is rate-limited by the reductive elimination of molecular hydrogen from a dihydrido iron species and not only exhibits a high regiospecificity but also occurs diastereoselectively. For the Fe+ complexes of heptanoic, octanoic, and nonanoic acids, an additional, unprecedented reaction channel evolves which leads to the unimolecular losses of the C-1-units CH3* and CH4. The dependence of the fragmentation patterns on the length of the alkyl chains of the carboxylic acids is well explained by the competition of all these reactions and reflects their different energy demands. With respect to the chain length effects, the scope as well as the limitations of the mass spectrometric method are discussed.