The formation and protonation of the complexes of acids N(CH₂COOH)_3 − r(CH₂PO₃H₂)_r, r = 1 (H₄ndamp) or 2 (H₅nadmp), with the alkaline-earth metal ions (M = Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺) and the divalent transition metal ions (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ or Cd²⁺) have been investigated by means of potentiometry and ³¹P NMR spectroscopy at 25 °C. The complex formation constants and protonation constants of these complexes were determined by pH titration. The ³¹P NMR spectra of the ligand and metal–ligand solutions were measured as a function of pH and the ³¹P NMR chemical shifts of each chemical species evaluated by using the equilibrium constants determined by pH titration. The stability constants of the metal complexes increase as the number of the ligand phosphonate groups (r = 0–3) increases. In the alkaline-earth metal complexes the first protonation takes place on the nitrogen atom except for Ca–ndamp and –nadmp complexes. In the transition metal complexes the formation constants follow the so-called Irving–Williams series although the Ni–nadmp and –ntmp (r = 3) complexes show a peculiar behavior. The change in the chemical shift of the complexes upon protonation supports the structures of the metal complexes predicted from the formation and protonation constants.