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The complexation of gadolinium(III) with citric acid in aqueous solutions was studied by pH-metric titration, proton magnetic relaxation, and mathematic simulation in the pH range 2.0–10 at [Gd 3+ ]: [H 4 Cit] = 1:1, 1:2, 1:3 ([Gd 3+ ] = 1.3, 2.6, 5.0, 10.0, and 20.0 mmol L −1 ). In the process of simulation, the equilibrium composition, a model obtained from previously known works and including  mono-  and  bis -citrate complexes of gadolinium(III) was taken as a basis. In this work, it is shown that a satisfactory description of the experimental data set of two independent physico-chemical methods is achieved only with the additional inclusion of citrate complexes of gadolinium(III) with higher degree of protonation and some new polynuclear complexes. The complex [GdH 4 Cit] 3+  with the molecular form of the citrate ligand, and the polynuclear complexes [Gd 2 (HCit) 2 ]°, [Gd 2 Cit 2 ] 2- , [Gd 6 (OH) 2 Cit 6 ] 8- , [Gd 6 (OH) 3 Cit 6 ] 9-  were first detected at the molar ratio of 1:1. At two- and three-fold excess of citric acid, mononuclear  bis - and  tris -citrate complexes with different degrees of protonation were found, and binuclear  tetrakis - and  hexakis -citrate complexes [Gd 2 (HCit)Cit 3 ] 9- and [Gd 2 (HCit) 4 Cit 2 ] 14-  at the pH &gt; 7.5. On the example of this manuscript, the importance of applying the NMR relaxation method for identifying polynuclear complexation in systems containing paramagnetic ions is shown.

New Aspects of Complex Formation in the Gadolinium(III)–Citric Acid System in Aqueous Solution