This chapter expounds further on the understanding of aluminum hydrolysis and polymerization in solution in the presence of charged surfaces. It reviews the influence of inorganic and organic anions on the surface reactions of adsorbed aluminum species. The chapter presents a discussion of the changes in chemical and physical properties of clays and soils produced by the adsorption of these species. Unbuffered salt solutions are believed to remove exchangeable aluminum ions bonded electrostatically to permanent charged sites, whereas buffered salt solutions will replace exchangeable aluminum as well as some fraction of nonexchangeable and precipitated forms of aluminum. It has been demonstrated that the nonexchangeable aluminum forms may have OH/Al ratios of <3.0. The fixation of hydroxyaluminum with a composition of approximately Al(OH)2+ (i.e., n = 2.0) by a Dowex® resin has been reported. Inorganic anions may also play an important role in governing the formation of three polymorphic structures, bayerite, gibbsite, and nordstrandite.