The protonation constants of tetraethylenepentaamineheptaacetic acid, TPHA, were determined by potentiometric titration in aqueous solution at an ionic strength of 0.10 M KNO3 and at 25°C. The formation constants of various metal-TPHA complexes were also obtained by titrating mixtures of metal to ligand in molar ratios of 1 : 1 and 2 : 1. Calculations were performed with the computer program BEST. Individual stability constants are reported for Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Hg(I1) and Pb(I1) with TPHA as well as their related protonated species. The stabilities of the 1 : 1 complexes parallel to those of similar complexes with DTPA and TTHA. However the 2 : 1 complexes have significantly larger log KML's than their TTHA counterparts. The extra stability of the 2 : 1 metal-TPHA complexes is explained in terms of ligand denticity and steric effects. Mercury(I1)-TPHA complexes exhibited the highest formation constants and the copper-TPHA complexes had slightly higher log KML's than those for Co(II), Ni(II), Zn(II), Cd(I1) and Pb(I1).