The adsorption of a cationic polyelectrolyte onto cellulose fibers in the presence of aluminum species was investigated. Solutions of either aluminum chloride or aluminum sulfate (alum) were used in the pH range 4.1-5.5. An apparatus was designed to simulate the mixing and turbulent conditions and to permit the short polyelectrolyte adsorption times found on typical paper machines. The effect of the aluminum species on the polymer adsorption depended on their concentration, counter-ion, pH, and time. From a knowledge of the equilibria of the aqueous aluminum species, it was possible to identify two distinct pH regions for aluminum and polyelectrolyte adsorption. At low pH, only soluble aluminum species were present and both aluminum salts reduced the rate and amount of polymer adsorption. At high pH the two aluminum salts formed distinctly different precipitates having very different effects upon polymer adsorption. The adsorption characteristics of each region are discussed and related to papermaking.