Using an improved description of the aqueous equilibria of the aluminum ion, a study was undertaken to improve the general understanding of the adsorption of complex aluminum species by cellulosic fibers under conditions of a papermaking system. The adsorption of aluminum for both aluminum chloride and aluminum sulfate was found to be primarily a function of the pH and aluminum concentration. At each aluminum concentration, the adsorption of aluminum was characterized by a sharp increase occurring between pH 4 and pH 5. This increase was determined to be primarily the result of a change in the aqueous solution chemistry of the aluminum ion, specifically the formation of a polynuclear complex species, and not the ionization of the cellulosic carboxyl groups. Using the aqueous equilibria, it was also possible to separate the aluminum adsorption process into three distinct pH regions. The adsorption characteristics of each region and its impact on the papermaking process are discussed. -- AATA