The adsorption of alizarin (AZ) and alizarin red (AZR) on anodic aluminum oxide (AAO) plates was studied by spectroscopic methods. AAO plates were prepared by anodization of aluminum plates in a 1.8 M sulfuric acid solution. Adsorption studies were carried out by immersing the AAO plates in ethanol solutions of AZ and AZR at different pH values, whereby the color of the solutions changed according to the pH due to protonation/deprotonation. The absorption spectra of AZ in solution and that adsorbed on AAO were different, indicating the formation of AZ-Al³⁺ complexes with 1:1 stoichiometry by coordination of AZ to the surface of AAO. Similar spectra were obtained for AZR adsorbed on AAO, confirming that AZR is also adsorbed on the surface by coordination and not via electrostatic interactions between the sulfonate group of AZR and the AAO surface. Based on the spectral features of the AZ-adsorbed AAO plates, the possibility of their application in pH sensing is proposed.