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 approximately 1 ps during the decay of the initially excited (1)pi pi * state. The quantum yields phi(D)(ps) determined after the (1)pi pi * decay equal the final yield phi(D)(cw). This gives evidence for a predominance of the singlet channel in CPD formation.