The dependence of the impedance Z and/or differential capacitance C of the electrode double layer on electrode potential E and on frequency in solutions of echinomycin with single stranded (ss) and double-helical (double stranded, ds) DNA was measured. Echinomycin gives a pseudocapacitance redox peak on the C-E curves at the potential of -0.53 V. This peak is observed with the dsDNA-echinomycin comlex as well, but not with ssDNA treated by echinomycin. The capacitance measurements can thus distinguish with the aid of bis-intercalator echinomycin the ss and ds form of DNA adsorbed at the electrode surface. This promise to use impedance measurements for detection of dsDNA in hybridization reactins. (cs)