The complex formation constants of two phosphonic acids, HEDP and ATMP, with three trivalent metallic cations, Al(III), Cr(III) and Fe(III), have been determined by acid-base titration at 25°C and constant ionic strength (0.1 mol l 1, KNO3), using Martell and Motekaitis' computer programs. Species distribution curves showed that all three cations are in complex form in the pH range of fresh waters (5-9). The study of different cation:ligand ratios proved that both ligands mainly form anionic soluble complexes for systems having an excess of ligand--as protonated and unprotonated forms and especially ternary complexes with HEDP. For higher metal concentrations (excess of cation), weakly soluble species of HEDP and ATMP were formed with Al(III) and Cr(III). Two insoluble complexes with ATMP have been identified by SEM:EDAX as AlH3X(s) and Cr2X(s). Regarding Fe(III) species, Fe(OH)3(s) precipitate seems to predominate in solution.