Protonated 4,4-dimethylcyclohex-2-enone, 1H, has been shown to isomerize on irradiation in FSO3H at −85 °C to give protonated 6,6-dimethylbicyclo[3.1.0]hexan-2-one, 5H, and protonated 3-isopropylcyclopent-2-enone, 4H. The protonated bicyclic ketone 5H was found to be both thermally and photochemically labile, in both instances rearranging to 4H. The first order rate constant for the thermal isomerization of 5H to 4H at −60 °C was 1.73 × 10−4 s−1. As 5H was shown to be photolabile in the presence of 1H, it was not possible to establish whether 4H was a primary photoproduct of 1H. No photoisomerizations of protonated cyclohex-2-enone and protonated 4-methylcyclohex-2-enone could be detected. The photoisomerization of protonated Δ1,9-10-methyl-2-octalone has been re-examined at low temperature in an attempt to detect tricyclic products.