The intercalation and photobinding of 8-methoxypsoralen with thymine in DNA is studied by computational chemistry techniques. Photochemical [2+2] cycloadditions at the S-1 surface display barriers of approximately 20 kcal/mol. The lowest lying excited surface has a defined potential well at the ground state TS, enabling for efficient decay. Avoided crossings between the S-1 and S-2 surfaces are also observed, which may influence the photochemical reactivity. The further reactions leading to crosslink formation from furan monoadducts but not from pyrone ones are fully explained by the excitation energies of the monoadducts, whereas the predominance of furan over pyrone monoadduct formation is not explained looking at the excitation energy surfaces alone. (C) 2009 Elsevier B.V. All rights reserved.