This work presents an original adsorption study of calf-thymus DNA molecules monitored by Electrochemical Impedance Spectroscopy (EIS) and Surface Plasmon Resonance (SPR) as a function of DNA concentration at a constant pH of 7.3 and room temperature (25±1°C). Since EIS measurements were carried out at open circuit potential, there was an adsorption process of DNA molecules at the electrode surface. This enabled C dl values to be obtained by using an equivalent circuit similar to the one proposed by Frumkin-Melik-Gaikazyan-Randles (FMGR). Values of optical film thickness (d opt ) of adsorbed DNA molecules were estimated by using SPR measurements. The variations of the d opt as a function of DNA concentration (c DNA ) can be related to transitions in the structural arrangement of the electrochemical double-layer, presumably caused by DNA conformational changes observed at two critical c DNA , i.e. c* and c e , known as the overlap and entanglement concentrations, respectively. A linear behavior was observed in each regime, in which the relation between d op and c DNA followed the power law according to: dilute regime (c DNA <c*): d opt ∼ c DNA 0.20 , semi-dilute regime without entanglements (c*<c DNA <c e ): d opt ∼ c DNA 0.79 and semi-dilute regime with entanglements (c e <c DNA ): d opt ∼ c DNA 0.57 . Through d opt and C dl values, a dielectric constant behavior as a function of DNA concentration was obtained, showing DNA transitions passing from diluted, semi-diluted unentangled and semi-diluted entangled solutions.