The complex formation of N-methyliminodimethylenebis(phosphonic acid)[MeN(CH₂PO₃H₂)₂, H₄midmp] and nitrilotrimethylenetris(phosphonic acid)[N(CH₂PO₃H₂)₃, H₆ntmp] with cobalt(III)–polyamine complexes has been investigated by ³¹P NMR and UV/VIS spectroscopies. The ³¹P NMR spectra of the reaction mixtures were measured at 0 °C as a function of pH. The analysis of the pH dependence of the ³¹P NMR signals revealed that two types of complexes were formed in the reaction of cis-[Co(en)₂Cl₂]⁺(en = ethane-1,2-diamine) with midmp: O-monodentate and O,O-bidentate having an eight-membered chelate ring. The N atom is not coordinated. The two types of complexes were separated by HPLC and characterized by ³¹P NMR and UV/VIS spectrometry. The protonation constants and ³¹P NMR chemical shifts of each protonated complex were determined and structures are proposed. The first protonation occurs on the imino nitrogen of midmp of the Co(en)₂ complexes. An intramolecular hydrogen bond plays an important role in the stability of the monoprotonated O-monodentate ligand complex. The complex cis-[Co(en)₂(NH₃)Cl]Cl₂ forms an O-monodentate ligand complex on reaction with midmp. The protonation constants and ³¹P NMR chemical shifts of this complex corroborate the structure of the Co(en)₂ O-monodentate midmp complex. The complexes of the cobalt(III)–polyamines with ntmp have the same structures as those of the corresponding midmp complexes.