Ultrapure aluminium was thermally evaporated onto various plastics (polyethylene 2.6-naphthalate, PEN; polyethylene terephthalate, PET; polyimide, PI and glass for comparison) and potentiostatically anodized in a citric buffer. The anodisation procedure was monitored coulometrically and each alumina film formed was characterized by impedance spectroscopy. The resulting anodic alumina films were amorphous (proven by X-ray diffraction, XRD) and acted as dielectric material in a solid state capacitor with Au top electrode. The capacitors characteristics were evaluated using IV curves and frequency domain measurements. The performance of the capacitors demonstrated low leakage currents and low dielectric losses. The contrary properties capacity and breakdown voltage could be chosen by selecting the anodisation voltage. For each substrate apparent oxide formation factors and capacities were determined coulometrically. The ratio between apparent formation factor and projected area allowed determining the surface roughness. This surface roughness together with the high purity aluminium films and the anodisation compression was responsible for the unexpected high mechanical stability of this composite material.