A relatively new mechanism for PAH activation to an active carcinogen is one-electron oxidation to produce a radical cation. Demonstration of this mechanism has employed fast atom bombardment and tandem mass spectrometry. Subject PAH carcinogens include benzo[a]pyrene, dibenzo[ ]pyrene, 7,12-dimethylbenzanthracene, and methylchrysene. Proof of mechanism is achieved by establishing structure of modified DNA bases and nucleosides. Future research is dedicated to improving sample handling and detection limits for demanding in vitro and in vivo experiments. A similar bioanalytical approach is under development for testing a new hypothesis that breast cancer is caused by oxidation of endogeneous estrogens and related steroids.