A comparison of the sulfate/hematite and the sulfate/maghemite adsorbate systems was carried out by the catalysis on aldol and retro-aldol reactions, FTIR and XPS. The chemisorption of the sulfate ions releases the initially adsorbed surface hydroxyl groups and thereby deactivates not only the basic but also the acidic active sites. The substitution takes place regardless of the nature of the surface hydroxyl groups. The adsorbed ions are strongly influenced by the local crystal structure of the adsorption site because the bridge-bond distances partially determine the sulfur-oxygen bond character. The bulk lattice properties are thus found to correlate to the adsorbate chemistry.