Continuous STFT transforms of a sinusoidal perturbation signal and current have been found for the first order electrode reaction. Electrode impedance has been determined in the joint time-frequency domain. Measurements of the Cd(II) reduction reaction have been performed on a dropping mercury electrode as a function of time. A possibility of instantaneous impedance spectra generation has been presented. Time characteristics of charge transfer resistance, Warburg coefficient and double layer capacitance has been described. (C) 2003 Elsevier Ltd. All rights reserved.