The formation and protonation of the complexes of a series of ethylenedinitrilopoly(methylphosphonic acids) (edmp) [(H₂O₃PCH₂)_2-pMe_pNC₂H₄NMe_q(CH₂PO₃H₂)_2-q; p, q = 0–2] with the alkaline-earth metal ions (M = Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺) have been investigated by means of potentiometry and ³¹P NMR spectroscopy at 25.0 °C. The complex formation and protonation constants of these complexes were determined by pH titration. The ³¹P NMR spectra were measured as a function of pH and the shifts of each chemical species evaluated by using the equilibrium constants determined by pH titration. The results for edmp complexes were compared with those of aminopoly(methylphosphonic acid) [Me_rN(CH₂PO₃H₂)_3-r, r = 0–2] complexes. The stability constants of the metal complexes increase upon increasing number of phosphonate groups and are almost the same among the edmp having the same number of such groups irrespective of the number of nitrogen atoms. These results indicate a structure where one nitrogen atom of the phosphonate does not co-ordinate in any of the complexes. The first protonation takes place on this nitrogen atom.