Polyacrylic acid (PAA), polymethacylic acid(PMA), and AA/MA copolymers were synthesized and the reaction of La(III) with then was investigated. The original polymer solution ($alpha$=0) was potentiometrically titrated with the fully neutalized polymer solution($alpha$=1) in the absence or presence of La(III) ions. From the data of polymer-La(III) system it was possible to calculate the number of carboxylate ions bound to one La(III) and to predict which polymer bind La(III) more easily at the same conditions. The results show that there is a specific binding between La(III) and polymeric acids(PAA, PMA, AA/MA copolymers) and that one La(III) binds three carboxylate ions so that a 3:1 complex is formed. The amount of La(III) bound to the polymeric acids is increased in the order of PAA > AA/MA copolymer > PMA at the same degree of neutralization. The IR spectrum of the polymer-La complex indicates that the binding between them is ionic. The complex has better thermal stability than the original polymer.