This chapter deals with the UV-induced fragmentation of mononucleotide and oligonucleotide ions isolated in vacuo. We review gas-phase photodissociation spectra of mononucleotides, single strands, double strands and quadruplexes, and compare these with the corresponding spectra in solutions to consider solvatochromic shifts. The role of multiple bases is considered within the Frenkel exciton model that describes the electronic coupling between two or more bases, which is relevant to understand photoexcitation. We discuss relaxation of the excited states and the time scales for nucleotide dissociation when there is no solvent quenching of the excess energy. Finally, the photophysical behaviour of DNA with respect to electron ejection will be discussed with an emphasis on the functional groups involved in electron loss.