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Quantitative acoustic measurements have been carried out on phosphoric acid-anodized (PAA) aluminum. This process creates a porous oxide layer which varied from 1 to 22 μm thick in the different specimens used here. The elastic properties of the PAA oxide layer were determined by measurements of the dispersion curves in the complex plane [speed of the surface acoustic wave (SAW) and its attenuation] alone. This was possible because two modes were observed on samples with thicker oxide layers. It has also been shown that the dependence of the attenuation on the oxide layer thickness around the cutoff can be used for the determination of the density of the layer. The shape of the attenuation curve measured by the acoustic microscope near the cutoff was smoother than predicted by the theory, and this phenomenon has been discussed.