Abstract The primary processes in the photoisomerization of 6-nitro-8-methoxy-2,2,3-trimethyl-2 H-chromene (Chr) in toluene were studied from −3 to 55 °C by measuring the transient absorption changes produced by neodymium glass laser excitation at 353 nm (third harmonic) with a full width at half-maximum of 6 ns. The results indicate that the formation of a long-lived (about 50 s at 24 °C) coloured isomer (I) occurs owing to CO bond opening, partly directly from the excited singlet state of Chr and partly via the triplet state. A novel result is the observation that bond opening in triplet Chr leads to a cisoid isomer in the triplet state ( 3X) which subsequently transforms into the coloured isomer. The lifetimes of 3Chr and 3X are about 80 ns and 450 ns respectively at 24 °C. The activation energies of the 3Chr → 3X and 3X → O reactions are about 8 kcal mol −1 and about 10 kcal mol −1 respectively. The use of Chr as a model system for studies of spiropyran photochromism is discussed.