The speciation and coordination geometries of M(III)-citrate complexes in aqueous solutions, where M denotes Eu, Tb, Lu, or Cm, are studied using potentiometric titration, 1H, and 13C nuclear magnetic resonance spectroscopies. Their photophysical properties are also characterized by time-resolved fluorescence spectra. The formation constants of mononuclear, dinuclear, and trinuclear Lu-citrate species, i.e., [Lu(Cit)2]3-, [Lu2(OH)2(Cit)2]2-, and [Lu3(OH)4(Cit)4]7-, were determined as 9.78 ± 0.30, 3.60 ± 0.30, and 1.02 ± 0.03, respectively, by potentiometric titration in 3.00M NaClO4 aqueous media. Terminal carboxylic conformation in trinuclear complexes comprised both the five- and six-membered rings at different exchanging rates. Hydration states evaluated for Eu3+ ions are the chemical formula of [Eu(Cit)2(H2O)2.5]3- and [Eu3(OH)4- (Cit)4(H2O)3.4]7-. These complexes in aqueous solution have geometrical similarity to the crystal structures in the literature. Furthermore, the entity of the hetero-trinuclear complex induces the intramolecular energy transfer from Tb3+ to Eu3+. The incorporation of Cm3+ into these homo/hetero-trinuclear citrate complexes proved to be a successful trial to probe the formation of actinide polymer at a trace level.