Three of the main oxygencontaining fragments resulting from 3-methyl-2-butyl trifluoroacetate (11) had been identified previously as the 1-trifluoroacetoxyethyl cation (m/z 141, 12, product of simple cleavage), and the products of single (m/z 142) and double hydrogen transfer (m/z 143, protonated ethyl trifluoroacetate). Collisionally activated dissociation of mfz 142 and the isotopmers resulting from ll-2-d, ll-l-d3, ll-5,6-d6, and 11-'80, has established that m/z 142 is tbe oxygen protonated 1-trifluoroacetoxyethyl free radical (17) formed by hydrogen shift from a y-methyl group to oxygen in the molecular ion, rather than in a complex (18) between 12 and the 2-propyl free radical, as expected based on a mechanistic model existing in the literature. The second hydrogen transferred originates in the other y-methyl group; its migration may occur, but does not bave to, in the complex between 17 and a molecule of propene, prior to dissociation of the two fragments. Collision-activated dissociation has now shown tbat the m/z 140 ion observed in the spectrum is the molecular ion of vinyl trifluoroacetate, possibly formed by a hydrogen transfer from 12 to the Zpropyl radical in the complex 18. The hydrogen migration to oxygen exhibits no isotope effect, whereas the transfers to carbon atoms exhibit small primary and a secondary kinetic isotope effects. Exclusive migration of the tertiary hydrogen from C(3) occurs in the formation of 2methylbutene cation radical (m/z 70) from the molecular ion. The hydrocarbon ion fragments and the heteroatomcontaining fragments are formed from 11 by disjoint pathways.