This chapter the summarizes information on the hydrolysis of the mononuclear aqueous aluminum ion and on the stability of mononuclear ion pairs or complexes of aluminum with naturally occurring inorganic anions and with organic ligands that are used as models for natural organic compounds or as important reagents in the colorimetric analysis of aluminum. The use of equilibrium thermodynamics to describe aqueous solution chemistry depends on the availability of data derived from accurate and precise experimental measurements, and on a reliable electrolyte theory for obtaining standard-state thermodynamic functions. Polarography, anode stripping voltammetry, and similar electrochemical techniques are generally inapplicable to the direct study of aqueous aluminum ion chemistry, due to the very negative reduction potential for Al(III). The hydrolysis of aluminum ion is the progressive loss of hydration shell protons to water molecules in the surrounding bulk solution, to maintain dissociation equilibrium.