UV-induced formation of cylcobutane pyrimidine dimers (CPD) in all thymine DNA models have been studied by femtosecond IR spectroscopy. CPDs are shown to form within 1 ps during the decay of the initially excited 1π π * state. The quantum yields Dps determined after the 1ππ * decay equal the final yield Dcw. This gives evidence for a predominance of the singlet channel in CPD formation.