The ring-opening reactions of perimidinespirocyclohexadienones in the ground and first excited singlet and triplet states were studied at the DFT B3LYP/6-31G**, UB3LYP/6-31G** and CIS/3-21G** levels of theory. It has been found that the thermal isomerization is governed by the non-adiabatic mechanism involving excitation in the triplet state. The irreversible character of the ring-opening photoreaction can be explained by the absence of a local minimum corresponding to the opened form at the S, potential energy surface. According to the TD DFT calculations the low intense long-wavelength absorption band at 436 rim inherent in perimidinespirocyclohexadienones belongs to a charge-transfer electronic transition involving the frontier orbitals located at different spiroconjugated moieties.