The focus of the presented study is electrochemical investigation of DNA heptamers d(GCNNNGC) with a different center trinucleotide sequence (NNN = GAA, AAA, CCC and GGG, where A, C and G is adenine, cytosine and guanine, respectively). In phosphate-acetate buffer at pH 5.8 all heptamers were adsorbed on a hanging mercury electrode and further reduced and oxidized. The voltammetric reduction peaks of A and C moiety and oxidation peaks of G moiety at different concentrations (0.25 to 10 µmol•L-1) were analyzed. It was found that the changes in the central trinucleotide sequence of heptamers result in altered adsorption ability at a charged phase interface. An electron transfer process in an adsorptive state was evidenced by the elimination voltammetric procedure (EVP). For the evaluation of adsorption efficiency of heptamers the Langmuir adsorption model was applied and the adsorption coefficient b was calculated.