The electrochemical behavior of nucleobases has been studied in 0.1 M phosphate buffer solution at pH 7.4, using a bare graphite electrode. Guanine and adenine produced well-defined oxidation peaks at about +0.63 and +0.91 V at 100 mV/s, respectively. Nucleobases exhibit an irreversible and hybrid-controlled electrochemical process, including adsorption and diffusion. The nucleobase oxidation peaks shift due to the selective interactions of nucleobases with each other. The oxidation peaks for three different pyrimidine bases, uracil, cytosine, and thymine, can be clearly identified at +1.26, +1.41, and +1.32 V, respectively. These differences in the electrochemical behavior among nucleobases can be attributed to their different chemical structures.