Three different electrode surfaces - two nanoporous and one modified with polyelectrolyte - provided with positively and negatively charged model proteins were studied at different conditions to determine characteristics of electrochemical impedance spectroscopy in biosensing. The impedance response in the presence of [Fe(CN)(6)(4-/3-) was simultaneously influenced by two effects - ionic attraction/repulsion and steric hindrance of protein components. The ionic interactions dominated at low ionic strength and at low concentrations of redox probe. On the other hand, the steric hindrance of immobilized proteins affecting mass transfer towards the electrode gained impact gradually with the increasing ionic strength and with the concentration of the redox-active component. The simultaneous effect of these two variables can lead to confusing results which are often misinterpreted when evaluating results from impedimetric affinity biosensors. (C) 2021 Elsevier Ltd. All rights reserved.