Transient resonance Raman and absorption spectra of the anion radical CM(.-) and lowest excited triplet state (3)CM* of coumarin were measured. Vibrational assignments of the ground state S-0, CM(.-), and (3)CM* were performed based on both the frequency shifts on isotopic (O-18, C-13, and deuterium) substitutions and normal-coordinate calculations using the force constants obtained by ab initio molecular orbital computations. The vibrational assignments and ab initio MO calculations provided much information on the structures of S-0, Chi(.-), and (3)CM*. It is concluded that the C(3)=C(4) bond of the pyrone ring is lengthened markedly in CM(.-) and drastically in (3)CM*. This suggests that the C(3)=C(4) bond is one of the reactive sites of coumarin and furocoumarins in their photochemical reactions, which is in good accord with the formation of cyclobutane-type adducts with pyrimidine bases, particularly with thymine, through the C(3)=C(4) bond of furocoumarins and the C(5)=C(6) bond of pyrimidine bases. The C=O bond was found to be moderately lengthened both in CM(.-) and (3)CM*. The moderate lengthening of the C=O bond of (3)CM* is consistent with the pi-pi* character of (3)CM*.