Carbonyl protonated aromatic ketones a—i [R—C(OH +)—C 6H 4CH 2OCH 3] containing a 4-methoxymethylphenyl group and R = H, CH 3, C 2H 5, n-C 3H 7, C 6H 5, 4-CH 3OC 6H 4, 4-CF 3C 6H 4, and CF 3, respectively, are generated in the ion source by 70eV electron impact induced dissociation of the benzylic alcohols 1—10. The ions a—i may dissociate to yiled acylium ions of different stability. The spontaneous fragmentations of metastable a—i have been studied by mass-analysed ion kinetic energy spectrometry and the reactions observed generally agree with the mechanisms suggested before for similar protonated aromatic ketones. In particular, these ions form intermediate ion/neutral complexes consisting of an acylium ion RCO + and a benzyl methyl ether molecule, and fragment by an internal ion/molecule reaction generating C 7H + 7 and RCOOCH 3 as well as by a direct dissociation releasing RCO +. By varying R it is shown that the competition between these fragmentations of metastable ions depends strongly on the stability of RCO +. In the case of carbonyl protonated benzophenones the proton migrates predominantly to the more basic (substituted) phenyl group. Labelling experiments show that a proton transfer between the two phenyl groups of the protonated benzophenones does not occur.