The complexes formed in the interaction between copper(II) anda-andβ-galacturonic acid, in the pH range 2.5–11.0, have been investigated by means of d.c. polarography and cyclic voltammetry. Witha-galacturonic acid, no complex is formed with copper up to pH 6. Between pH 6 and about 9.5, a complex is formed in solution. Above pH 9.5, the complex appears to break up releasing the ligand. In the case of β-galacturanic acid, no complex is formed until pH 3.5, and persists in solution up to a pH of about 9.5. A second complex forms above pH 6.9 and co-exists with the first complex up to pH 9.5. The complexes formed with both forms of galacturonic acid were studied and the stability constant of the coppera-galacturonate determined.