Abstract Guanine, having lower one-electron oxidation potential than other nucleobases, is of relevance to oxidative degradation of nucleic acids in mutagenesis, carcinogenesis, and aging. Here we compare oxidation potentials of guanine (G), guanosine (Guo), deoxyguanosine (dGuo), guanosine -5′- monophosphate (GMP) and 2′- deoxyguanosine -5′- monophosphate (dGMP) obtained by theoretical and experimental methods. Structures of G species were optimized and the identities of minima were verified by vibration frequency calculations. Redox equilibria were modelled in terms of corresponding thermochemical cycles. The changes in free energy were calculated at DFT level using the two different functionals: (i) general purpose B3LYP functional, and (ii) more specific ωB97X-D functional (both with 6-31 + G(d) basis set). Experimental oxidation potentials of all G analogues were measured voltammetrically on a polymer pencil graphite electrode (pPeGE) providing the best results from all carbon electrodes used (glassy carbon electrode, basal and edge plane pyrolytic graphite electrodes). The oxidation process is strongly dependent on the pH value and with increasing pH a linear shift of G oxidation peaks (Epa) towards negative potentials is observed. The theoretically and experimentally obtained oxidation potentials were compared for the pH 5. Anodic peak potentials increase in the order G « dGMP ≤ GMP