On the basis of spectroscopic and wave-function analyses of the excited states of psoralens, it is predicted that the photocycloaddition at the 3,4-double bond leading to the formation of a monoadduct proceeds from both the singlet and triplet excited states, with the latter state being the preferential route, especially in the reactions between psoralens and nucleosides and nucleotides in solution. The photocycloaddition at the 4',5'-double bond is likely to proceed by way of the singlet path exclusively. It is also predicted that the second photocycloaddition of the 3,4- or 4',5'-monoadduct yielding cross-linked diadducts takes the singlet route.