The electrochemical oxidation of guanine monolayers at the graphite-water interface was studied with scanning tunneling microscopy (STM), atomic force microscopy (AFM), and electrochemical methods. The oxidation was found to start preferentially from line defects. The use of a potential pulse method allowed us to study processes faster than the time scale of STM/AFM (due to limited scan rate). The oxidation speed and the number of electrons per molecule involved in the oxidation were directly determined. The detailed oxidation processes revealed by AFM enabled us to propose a model for interpreting the electrochemical measurements.