In this paper we investigate the tensammetric responses of different synthetic oligonucleotides (ODNs) by an alternating current (AC) voltammetry at the platinum solid amalgam electrode (PtAE). The optimisation of the AC voltammetric measurement of ODNs includes the determination of the effects of: adsorption time (t(A)), concentration of analysed sample (c), thickness of the amalgam layer (h), and use of different solid amalgams such as platinum, copper, gold and silver on the intensity of the AC voltammetric peaks 1 (desorption of ODN segments adsorbed via ODN sugar-phosphate backbone) and 3 (desorption and/or reorientation of the ODN segments adsorbed through hydrophobic base residues) of adsorbed ODNs. The features of mercury modified graphite/carbon electrodes for measurements of tensammetric responses of ODNs are briefly discussed. We show that from the height of the AC voltammetric peak 3 and potential difference between the AC voltammetric peaks 1 and 3 it is possible to identify: (a) different homopurine ODN lengths, and (b) a proportion of purine (adenine and/or adenine + guanine) units to the whole length of different ODNs containing both the purine and pyrimidine units. (c) 2006 Elsevier Ltd. All rights reserved.