Electrochemical impedance spectroscopy (EIS) is a versatile technique to characterize electrochemical systems satisfying causality, linearity and stationarity requirements. Odd Random Phase multisine EIS (ORP-EIS) can measure time-varying impedances for non-stationary processes. In this work, operando ORP-EIS is introduced to monitor the anodizing of aluminum, which is a nonlinear and non-stationary process, by superimposing a multisine AC perturbation signal on the anodizing DC potential. Results show that the multisine signal does not disturb the growth of the oxide film. Moreover, this technique makes it possible to follow the growth of the barrier oxide layer and provides an opportunity to investigate processes like ion adsorption, ion incorporation and ion movements through the oxide layer during anodizing. These processes cannot be studied by conventional EIS.