The photoisomerizations of protonated 5-methylhex-3-en-2-one and 4-methylpent-2-enoic acid have been examined. In each case a relatively rapid cis/trans isomerization about the carbon–carbon partial double bond and a slower conversion to protonated dihydrofuran 2H or protonated lactone 5H, respectively, were observed. The transformation of the acyclic cations to the five-membered ring cations was shown to occur by a photo-initiated, intramolecular hydride shift from the γ to β carbon atom. This shift is fully in accord with an excited state twisting/charge localization model for the excited states of these systems. Control experiments and the thermal chemistry of these ions is described.