Protonated methoxyhexenes exist as hydrogen-bonded structures in the gas phase. The hydrogen bonding facilitates exchange between the proton of the ether group and the hydrogen atoms of the hexenyl chain that precedes dissociation by loss of methanol. The position of the double bond has an effect on the formation of cyclic intermediates and the mechanisms for hydrogen transfer in metastable ions and following collisional activation. Collisional neutralization of protonated methoxyhexenes results in complete dissociation of the intermediate radicals by cleavage of the CH 2 -O and O-H bonds, but not the O-CH 3 bond. A mechanism for intramolecular hydrogen atom trapping by the double bond in the radicals is suggested. Collisionally activated dissociation and neutralization-reionization mass spectra of isomeric dimethylsilyloxy-hexene cations provide little structural information as to the position of the double bond.