Measurements ofk*Cg, the complex dielectric constantk*times the geometrical capacityCg, of a metal‐oxide‐plasma capacitor have shown thatk*exhibits dispersion in the audio frequency range. A series of dielectric relaxation spectra has been obtained for Al2O3layers (anodized) by varying the plasma density. The dependence of the spectra on plasma density and oxide thickness is indicative of Maxwell‐Wagner behavior. A Cole‐Cole representation of the data shows a broadening of the dispersion toward low frequency with a secondary dispersion process appearing at low frequency. The main dispersion can be described by an empirical dispersion formula which represents an asymmetrical distribution of relaxation processes. The secondary dispersion may result from a very low frequency dispersion of the admittance components of the plasma sheath or from charge migration in the oxide.