 Guanine plays an important role in many biological processes since it constitutes the buildings blocks of DNA and RNA. Solutions of guanine were prepared to cover the range from 5 × 10−5 to 8 × 10−4 M. Adsorption of guanine on a mercury electrode in acetic buffers at pH 4 and pH 6 is described by means of the adsorption isotherms constants calculated from the surface pressure as a function of electrode potential and adsorbate bulk concentration. The adsorption parameters from the double layer were calculated based on the data from the differential capacity–potential curves. The entirely different change in the differential capacity in two applied acetate buffers in the absence of guanine results from different buffer composition, as in a solution with a pH 4 adsorption primarily involves acetic acid molecules, whereas in a buffer with a pH 6-acetate ions. The possibility to accurately determine Emax and σmax parameters points to a physical character of guanine adsorption, which must be associated with the fact that adsorbed guanine molecules are vertically or diagonal oriented. Obtained RA (charge transfer resistance at formal potential) changes are minor and comply with changes in ΔE. Therefore, it may be concluded that in the tested range of guanine concentrations, it has no effect on kinetics of zinc cation depolarisation at a mercury electrode both in the buffer with pH 4 and pH 6.