Time resolved impedance spectra measurements were performed on potential-stepped Ag electrodes using a FFT impedance spectrometer which operates with a sum of properly defined sine waves as a perturbation. New frequencies were observed in the current response spectra in cases when current relaxation had not decayed before the perturbation was applied. The analysis of the amplitude spectra of the response current allows one to determine whether the obtained impedance spectra were "good" or "bad". Possibilities are outlined for the correction of the measured current data in order to obtain valid impedance spectra.