The voltammetric behaviour of the pyrimidine bases thymine and cytosine is studied using a glassy carbon electrode. In contrast to previous reports that assume electroinactivity at carbon electrodes, it is demonstrated that both compounds undergo well-defined oxidation at a glassy carbon electrode. The experimental conditions that influence the electrode reaction, in particular the pH of the electrolyte solution, are varied systematically and optimized for voltammetric determinations. The application of ultrasound in combination with differential pulse voltammetry results in a reliable analytical procedure for thymine and cytosine measurements avoiding electrode fouling and maintaining the electrode characteristics. The effect of ultrasound is mainly to enhance transport of electroactive species and to clean the electrode in situ. Besides studying the sonovoltammetric behaviour of cytosine and thymine separately, simultaneous determinations are also performed and extended to the analysis of adenine, guanine, thymine and cytosine in the same solution.