The present investigation addresses the correlation between chemical and mechanical properties of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) used as an interface layer in bioelectronic devices. This correlation is important for the functional characteristics of PEDOT-based biosensors including neural electrodes. Electrochemical polymerization of PEDOT is studied in aqueous solutions of lithium perchlorate, sodium polysterenesulfonate(PSS) or sodium dodecylsulfate(SDS). Simultaneous electrochemical and acoustic impedance measurements are carried out in the course of polymerization and voltammetric cycling in acidic (0.4 M HClO4) and neutral (0.1 M sodium phosphate buffer) solutions. The acoustic impedance results are used to obtain data for the complex mechanical shear modulus, G, of the three types of PEDOT and to follow the ionic and solvent transport in the course of their electrochemical oxidation/reduction. It is established that the doping anion significantly affects the mechanical properties of PEDOT with PEDOT/dodecylsulfate displaying significant stiffness. PEDOT/perchlorate takes an intermediate position whereas PEDOT/PSS has the lowest G values. This trend in mechanical properties is preserved upon voltammetric cycling in both neutral and acidic media with marked increase in the stiffness observed in 0.4 M HClO4 for the PEDOT/dodecylsulfate case. It is demonstrated that the type of ionic transport (anionic or cationic) and the amount of solvent molecules involved in the redox process depend on both the availability of immobilized negative charges (PSS or dodecylsulfate) and on the hydrophilicity of the PEDOT layers. The presented results point to versatile possibilities for controlling the mechanical properties of PEDOT in a way to match them to device-specific requirements. (C) 2015 Elsevier Ltd. All rights reserved.