Abstract: Stability constants and fractional distribution of europium(III) ion complexes with aminotrismethylenephosphonic acid in aqueous solutions at 25°C were determined potentiometrically in numerous systems with various metal-to-ligand ratio namely 1 : 1, 1 : 2 and 1 : 4. This approach allowed to accurately describe equilibrium process by set of complexes including previously unreported Eu2(Н2L)2, Eu2Н3L2, Eu2(НL)2, Eu2НL2 forms. The dimerization is thermodynamically favorable for complexes of all protonation degrees. Apparently the dimerization is a way to achieve the most advantageous arrangement of positively charged europium ions and negatively charged phosphonic groups. Eu : ATMP = 1 : 2 complexes are formed in excess of the ligand as a result of a thermodynamically favorable process, but the formation reactions of EuL2 и Eu(НL)2 species seem to have kinetic restrictions. The luminescence spectroscopy method was applied for testing proposed complexation model. We demonstrated that obvious differences in the fine structure of spectra are observed between the spectra of solutions containing predominantly dimeric forms Eu2HxL2 and the spectra of solutions containing predominantly bis-complexes EuHxL2. Thus, the results of the luminescence spectroscopy are in good agreement with the results of potentiometrictitration in the europium(III)–ATMP system.