Abstract The complex species formed in aqueous solution between 2,5-toluenediamine-N,N,N',N'-tetraacetic acid (2,5-TDTA) and Cu(II) were studied at 25°C and ionic strength 0.1 M in KC1. Potentiometric investigations were carried out at ligandrmetal ratios 2:1 and 1:1 (CM = 1.5 mM and 2mM), and ratios 1:3, 1:4 and 1:5 (CL = 2mM). The applications of the least-squares computer program LETAGROP to the experimental potentiometric data, in the pH range studied (2.29-8.74), indicate that the following complex species are formed (ligand H4C): [CuHC]-, [CuC]2-, [Cu(H2C)2]2-, [Cu(HC)2]4- and Cu2C. 2,5-TDTA acid, as does para-phenylenediamine-N, N, N, N'-tetraacetic acid (p-PDTA), forms complexes with excess of ligand in the ratio 1:1, or with excess metallic cation. This is due to the fact that the diaminepolycarbox-ylic acid derivatives of meta or para-phenylenediamines can only coordinate an atom of nitrogen to any one metallic cation. The values of log K indicate the order of coordinating capacity p-PDTA > 2,5-TDTA. This order is attributed to repulsive steric effects between the methyl radical and the metallic cation.