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Oxide films grown anodically at the surface of superpure aluminium are used as standards to assess the accuracy of the thickness (d_XPS) determined using the Beer–Lambert treatment of the Al 2p metal and oxide peak intensities. For the fitting conditions employed, the value of d_XPS is found to be very close to the true film thickness for films <10 nm thick, beyond which imprecise values are obtained. A surface layer of hydration is identified from the curve fitting of the O 1s core level, which would qualitatively account for the slight underestimate of film thickness provided by the expression for d_XPS. The validity of the Al 2p fit may be determined through this correlation of thickness but requires first a quantitative assessment of the thickness and composition of the hydroxide‐rich layer. Copyright © 2002 John Wiley & Sons, Ltd.