Oxidative damage to DNA is implicated in mutation, premature aging and disease. At the molecular level, the primary process is one-electron oxidation of one of the nucleobases to form a radical cation. Despite the fundamental importance of the nucleobase radical cations, most of what is known about their production and reactions in duplex DNA has come from indirect experimentation. Specifically, direct spectroscopic studies have proved difficult due to small signal sizes, interfering absorptions from reaction initiators, and low time-resolution in existing methodology. This thesis describes a novel approach to the study of the radical cations of the DNA nucleobases. Several novel photochemical sensitizers have been developed that allow these species to be formed with high efficiency, with high time-resolution, and with minimal interference in spectroscopic experiments. This approach has allowed the reactions and properties of the nucleobase radical cations to be studied both in buffered water and in duplex DNA. The kinetics of electron and proton transfer reactions of the purine radical cations are described for the first time. The thermodynamics of these electron and proton transfer reactions have also been determined using novel kinetic methods. Studies in duplex DNA have shown that rapid deprotonation of the guanine radical cation occurs, shifting the location of the positive charge from guanine to cytosine, which has significant implications for the mechanisms of long-range charge transport and formation of DNA oxidation products. Experiments in DNA with multiple G sequences suggest that deprotonation can be inhibited due to charge delocalization. Experiments in DNA with adenines and thyrnines allow observation of both purine and pyrimidine oxidation intermediates. Spectroscopic evidence for formation of delocalized radical cations is obtained for DNA containing multiple A sequences. These experiments represent the first direct spectroscopic studies of DNA oxidation.