A series of substituted 2,3-dihydroxyterephthalamides has been prepared and investigated by means of potentiometry, spectrophotometry and 1H NMR spectroscopy at 25 °C. Proton association constants for the ligand and formation constants for the ferric complexes are as follows: 2,3-dihydroxy- N,N’-diethylterephthalamide, log β 011 = 10.83, log β 012 = 17.09, log β 110 = 17.27, log β 120 = 31.59, log β 130 = 42.67; 2,3-dihydroxy- N,N’-bis(2-hydroxyethyl)terephthalamide, log β 011 = 10.61, log β 012 = 16.66, log β 110 = 17.04, log β 120 = 31.0, log β 130 = 42.1; 2,3-dihydroxy- N,N’-bis(ethanesulfonate)terephthalamide disodium salt, log β 011 = 11.01, log β 012 = 17.48, log β 110 = 18.24, log β 120 = 31.72, log β 130 = 40.3; 2,3-dihydroxy- N, N’-bis( N, N-dimethylaminoethyl)terephthalamide dihydrobromide salt, log β 011 = 10.86, log β 012 = 20.10, log β 013 = 28.53, log β 014 = 33.92, log β 110 = 17.4, log β 120 = 32.5, log β 130 = 45.2. The equilibrium free metal ion concentrations (–log 〚Fe 3+〛 = pM) of these ligands are compared and discussed.