The adsorption isotherms of poly(acrylic acid) (PAA) onto alumina powder have been determined as a function of pH, ionic strength, and magnesium-ion concentration. The adsorption of PAA is strongly enhanced by magnesium ions in alkaline media but less affected under acidic conditions. The adsorption isotherms display a maximum when PAA is fully complexed with magnesium ions in the solution, corresponding to a ratio of 0.25 +/- 0.05 [Mg2+]/[acrylic acid monomer]. The decrease in adsorbed amount with an increase in PAA concentration at constant magnesium-ion concentration is related to a decrease in the complexation ratio.