AbstractAn attempt is made to estimate the accuracy which can be achieved in the evaluation of parameters of an electrode with an activation controlled electrochemical process from measurements of electrode impedance. Ferrous/ferric couple at platinum in perchloric acid was taken as a model system and the impedance components have been measured as functions of frequency at different concentrations. Double layer capacitance, CDL, faradaic resistance, θ, and the ohmic resistance of the cell R, have been identified from the characteristic functions, and the indeterminacies in their values evaluated from the probable errors of measurements. The problem of indeterminacy was generalized and regions in the CDL – θ – R space were defined outside which those parameters cannot be determined within present limits of accuracy.