Abstract— The lowest excited singlet and triplet states of coumarin, psoralen, and 4‐hydroxy‐coumarin have been assigned to the (π,π*) type on the basis of the luminescence spectroscopy and MO calculations. The mechanism of photocycloaddition of courmarin and psoralen to thymine has been described in terms of the perturbational MO model and MO reactivity indices. All possible cycloaddition patterns have been examined. Results suggest that the 3,4‐bond of coumarin in the excited state is somewhat more reactive than the same bond of psoralen in the excited state. It is also predicted that the 3,4‐bond of psoralen in the triplet state is more reactive than the 4′, 5′‐bond. The results have been favorably correlated with the electronic characteristics of excited coumarin molecules and with available experimental data on the relative yields of photoadducts.