Ground state and triplet π → π* states of canonical Watson–Crick base pairs have been studied at the B3LYP level of theory. Excited states were found to be localized at either of the monomers forming the base pair (guanine, cytosine, adenine and thymine), geometry relaxation of the excited base pair being similar to that occurring in the isolated nucleobase. For thymine and cytosine, triplet π → π* excitation produces a significant elongation of the C5–C6 bond whereas for guanine and adenine there is a significant increase of the N3–C2 bond and pyramidalization of the NH2 group. Adenine–thymine energy pairing remains almost unaffected by triplet excitation. However, for guanine–cytosine, with excitation localized at the guanine moiety, base pairing energy decreases about 5 kcal/mol due to pyramidalization of the amino group of guanine. ∥Dedicated to Professor M. Robb. © 2006 Taylor & Francis Group, LLC.