Electrochemical Impedance Spectroscopy can be used very effectively for studies regarding the passivity of metals. In general there are two different methods to interpret measured impedance data. The first method uses electrical equivalent circuits. Alternatively a transfer function can be calculated, ie a theoretical impedance expression based on an assumed physical model. The latter method is a more fundamental approach but often corroding systems including passive alloys are too complicated to arrive at useful transfer functions. In that case the impedance data are in general analysed according to the first method. An equivalent circuit is fitted to the measurement data. For some cases the transfer function can be derived and analysed, when accepting some approximations. As an example the passive behavior of aluminium will be discussed.