Cytosine plays an important role in many biological processes since it constitutes the buildings blocks of DNA and RNA. A two-step reduction of Zn2+ ions at the dropping mercury electrode in acetic buffers at pH 4 and 5 in the presence of cytosine was examined. The measurements were performed using an impedance method in a wide potential and frequency ranges. The values of the standard rate constants k(s) in the both studied system decrease from 3.8 x 10(-3) to 2 x 10(-3) cm s(-1) at pH 4 and from 5.1 X 10(-3) to 2.5 x 10(-3) cm s(-1) at pH 5. The values of the standard rateconstants ks, characterizing the stage of the first electron transfer decrease similarly. However, the values of the standard rate constants k(s2) characterizing the stage of the second electron exchange decrease more markedly in the buffer at pH 4 than in the buffer at pH 5. (c) 2005 Elsevier Ltd. All rights reserved.