Theoretical principles for determination of instantaneous impedance spectra were formulated. Coupling of the pseudo-white noise and short-time Fourier transformation methods allows determination of impedance spectra changes as the function of time. The theoretical principles of the new measurement method indicate the possibility of conducting impedance investigations of non-stationary processes. The frequency resolution and time selection of the total time-frequency analysis of impedance spectra depend on the choice of the size of the Gaussian window.