DNA methylation is an important epigenetic event playing crucial roles in physiologic and pathologic processes. We show that methylation of cytosine (C) residues in DNA can be easily detected electrochemically using mercury or solid amalgam electrodes. Reduction peaks of untreated single-stranded methylated and non-methylated oligodeoxynucleotides (ODN) do not significantly differ. Using DNA bisulfite treatment, reducible Cs are transformed into nonreducible uracil residues, strongly decreasing square wave voltammetric C reduction peaks. On the other hand. 5-methylcytosine (mC) residues resist the bisulfite treatment and display almost unchanged reduction peak. Desulfonation step should be omitted because uracil sulfonation improves the resolution of C from mC. By combining DNA bisulfite treatment with square wave voltammetry. DNA methylation can be determined quantitatively at nanomolar and subnamolar ODN concentrations. (C) 2012 Elsevier Ltd. All rights reserved.