Equilibria between Al3+ and the polyelectrolyte polyacrylic acid (PAA) were studied in 0.1 M NaCl medium at 25 A degrees C (M = mol dm(-3)). The measurements were performed as precise potentiometric titrations in which OH- was in most cases generated coulometrically. The total concentration of Al(III) and PAA varied within the limits 6.23 x 10(-5)-2.01 x 10(-4) M and 7.20 x 10(-4)-1.12 x 10(-3) M, resp., with a 4-18 fold excess of ligand. The value of - log{[H+]/mol dm(-3)} varied between 3 and 5-7, the upper limit set by drifting EMF potentials caused by the onset of a precipitation reaction. The constant capacitance model was utilized to model the experimental data. With the objective of finding a model as simple as possible that gives an acceptable fit to data, the two species AlPAA(2+) and Al(OH)(2)PAA are postulated. The polyelectrolyte effect resulting in pH-dependent formation constants is demonstrated. Furthermore a comparison between the stability constant of the 1:1 complexes of Al3+ with acrylate(-) and PAA(-), respectively, shows the logK value of the latter to be five units higher and is ascribed to particular properties of the polyelectrolyte.