This paper reports the characterisation of chemical conversion layers on aluminium by means of Electrochemical Impedance Spectroscopy. To fit the EIS spectra non-trivial dispersion phenomena, which are usually described by means of a constant phase element taking into account fractal dimensions or porosity effects, have to be considered. By investigating the influence of the nature and the conductivity of the EIS electrolyte on the impedance behaviour, the observed dispersion can be attributed to a ladder-network of electrolyte resistances and double layer capacitors in the pores. The proposed electric equivalent circuit allows to study the growth of the film as a function of the conversion parameters. In this paper the influence of the conversion time on the film thickness is worked out. In agreement with AES, SE and FTIRS the layer growth is in proportion to the conversion time.