Protonated 2,4-hexadienal (1H) and 2,4,6-octatrienal (2H), prepared by protonation of the analogous aldehydes in FSO3H, isomerized to give cyclized products 3H and 4H at 30 °C and −20 °C respectively. The rate constants for the cyclization of 1H were measured in both FSO3H and CF3SO3H. It was found that the rate constant for isomerization decreased when CF3SO3H was used as the reaction medium. It is suggested that the thermal cyclizations of 1H and 2H involve diprotonated species with protonation occurring on oxygen. 1H and 2H underwent photoisomerization at −78 °C in FSO3H to yield 5H and 7H respectively. It was found that 5H underwent a cyclization reaction at −40 °C to give 6H, which subsequently rearranged to give 3H at −10 °C. However, 7H was found to isomerize to 2H at −50 °C. The mechanisms of the thermal isomerization are discussed.