Several important applications of pure aluminium sheets (corrosion resistance, adhesion of polymer coatings, etc.) are strongly dependent upon the water content of the thin oxide layer naturally grown during processing and storage of the material. A characterization approach is presented based on capacitance measurements over a wide frequency range (4kHz–10mHz) at the aluminium/oxide/electrolyte (aqueous or nonaqueous) interface. It is established that the higher the water content of the film, the larger the frequency dispersion of its capacitance. Systematic processing of a wide body of complex capacitance data (Cole-Cole plot) allowed the unambiguous demonstration that the layer capacitance consists of two contributions in series: (1) the capacitance of the bulk oxide obeying a CP behaviour depicted by a Jonscher relationship. (2) a capacitance attributed to the oxide/electrolyte interphase. Taking into account many experimental parameters such as solvent and electrolyte composition, temperature, water content of the layer, the origin of the frequency dispersion and of its water content dependence is discussed.