This chapter reviews the surface chemical behavior of aluminum oxides and hydroxides, with an emphasis on adsorption studies and the electrochemical behavior of the alumina–water interface. The coordination behavior of aluminum cation toward hydroxide ions explains the marked dependence of aluminum hydroxide solubility on pH. Thus, the initial surface structure can be related to the crystal structure and the way in which the surface was formed. Furthermore, the surface chemical behavior of aluminum oxides and hydroxides can be explained in part by surface structure expected for particular crystals. All strongly dried aluminum oxides and hydroxides chemisorb at least a monolayer of water when exposed to moisture at room temperature. Defects and elemental substitutions in the gibbsite crystal structure may lead to a net charge imbalance. The surface acidity (ability to release protons from surface sites) of aluminum oxides is of long-standing interest, for both practical and theoretical reasons.