The complex formation equilibria of Cr3+ and Fe3+ ions in aqueous solution with two biodegradable amino-polycarboxylate chelants (DL-2-(2-carboxymethyl)nitrilotriacetic acid (GLDA) and 3-hydroxy-2,2'-iminodisuccinic acid (HIDS)) were investigated. The potentiometric data obtained at the constant ionic strengths (I) of (0.1 and 1.0) mol.dm(-3) KCl and at (25 +/- 0.1) degrees C was processed with the aid of the computer program HYPERQUAD 2008. The formation constants of the proton-chelant and metal-chelant (log K-ML) species (M = Fe3+ or Cr3+; L = GLDA or HIDS) were determined, and the concentration distributions of complex species in solution were evaluated for both metal ions. In various pH conditions, the interaction between the chelants (L = GLDA or HIDS) and the metal ions (M = Fe3+ or Cr3+) leads to the formation of different complexes formulated as MH2L+, MHL, ML-, M(OH)L2-, and M(OH)(2)L3-. The log K-ML values at I = 0.1 mol.dm(-3) KCl (T = (25 +/- 0.1) degrees C) were 15.27 (log KFe-GLDA), 14.96 (log KFe-HIDS), 13.77 (log KCr-GLDA), 12.67 (log KCr-HIDS), and at I = 1.0 mol.dm(-3) KCl (T = (25 +/- 0.1) degrees C) were 14.79 (log KFe-GLDA), 14,34 (log KFe-HIDS)) 12.90 (log KCr-GLDA), 12.09 (log KCr-HIDS). The e conditional stability constants (log K'(ML)) of the ML complexes were calculated in terms of pH in the range of 2 to 12 and compared with the same for EDTA and other biodegradable chelants (NTA and EDDS).