Abstract The photochemical pericyclic ring opening of the new photochromic system of spiro[1,8- a]dihydroindolizines (DHIs) was studied. From absorption and emission spectra the singlet energy E(S 1) of the DHIs was calculated to be of the order of 60 – 70 kcal mol −1. Measurements of the fluorescence quantum yields φ F and the singlet lifetimes τ s of the DHIs (at room temperature and 77 K) as well as quenching experiments gave radiative lifetimes τ rs which are two orders of magnitude larger than the τ rs values calculated by the Strickler—Berg equation, suggesting that the two excited singlets involved in absorption and emission are not the same. From negative reaction quenching and negative chemiexcitation by dioxetane the excited state of the DHIs was concluded to be a singlet. The pericyclic ring opening may therefore be regarded as a fast singlet reaction.