Many mechanisms have been advanced for the formation of alkyl-radical-loss ions in the mass spectra of fatty acid derivatives. Murphy has shown that these are the dominant products under dissociation conditions of lowest energy. With use of unit resolution spectra measured with low mass discrimination, H-2 and C-13 labeling evidence is only consistent with the Longevialle mechanism involving *C(OH)OCH3+ migration, with a preference for a 6-membered-ring intermediate (Scheme 1). This mechanism also predicts formation from the linear precursor of a branched product, the protonated 2-alkylpropenoate ion, as confirmed by MS/MS/MS using mass spectra from collisionally activated dissociation of this product. Other alky-loss products formed at higher energies appear to be due to this plus competitive mechanisms based on earlier proposals.