A model of deterministic and stochastic behaviour of electrode—electrolyte interface where electroactive species are strongly adsorbed is presented. It leads to the current—voltage curve, the electrochemical impedance and the electrochemical noise. An experimental study of an electrode coated with a thin film of redox polymer shows a good agreement between the model predictions and the experimental data, and leads to an experimental confirmation of the Fluctuation—Dissipation theorem (Nyquist theorem) for this system at thermodynamic equilibrium. The kinetic parameters are deduced from the impedance.