Femtosecond pump-probe spectroscopy was combined with photoelectron-photoion coincidence detection investigate the electronic structure and dynamics of isolated adenine (A) and thymine (T) dimers and the adenine-thymine (AT) base pair. The photoelectron spectra show that pi pi* and n pi* states are only weakly perturbed in the hydrogen-bound dimers as compared to the monomers. For cationic base pairs with internal energies greater than 1 eV, we observed considerable cluster fragmentation into protonated monomers. This process selectively removed signals from the n pi* -> n(-1) ionization channel in all dimers. The photoelectron spectra are compared to time-resolved mass spectra and confirm the assignment of short-lived pi pi* and n pi* populations in the adenine, thymine, and mixed AT dimers.