This study investigates how rare earth elements (REEs), such as europium (Eu), bind to organic matter. We are also gaining valuable insights into how these elements affect the structure of the organic matter that controls their mobility in natural systems, helping us to better understand the broader processes that govern the behaviour of trace metals in the environment. Humic substances (HS), including humic (HA) and fulvic (FA) acids, play a vital role in environmental systems, particularly in the sequestration and transport of trace metals. Although existing models like the NICA–Donnan model have illuminated metal–HS interactions, the effect of HS concentration on these interactions remains insufficiently explored. This study centres on Suwannee River fulvic acid (SRFA) and investigates how its concentration influences the dimensions and electrostatic characteristics of SRFA complexes, utilising europium(III) (EuIII) as a representative metal cation. The employed methodology involves Taylor dispersion analysis (TDA) with capillary electrophoresis to determine diffusion coefficients (D) and hydrodynamic radii (RH) of SRFA and EuIII–SRFA complexes. The NICA–Donnan model is employed to estimate site densities, intrinsic heterogeneity and Donnan potential (ψD). The RH values for SRFA and EuIII–SRFA complexes consistently fall between 0.78 and 1.03 nm, indicating that pH and SRFA concentration minimally affect complex size. Donnan volume calculations based on RH align well with the NICA–Donnan model. The results reveal changes in electrostatic properties, particularly the Boltzmann factor (χ), which is sensitive to SRFA concentration, exhibiting more pronounced effects in trivalent cation-containing SRFA complexes. The study underscores that the concentration of HS, such as SRFA, significantly influences the size and electrostatic attributes of SRFA complexes, potentially affecting the behaviour of other cations in environmental systems, including iron and aluminium. The findings highlight the importance of the metal to HS sites ratio in determining complex size and electrostatic properties, providing valuable insights into the behaviour of natural organic matter in diverse environmental contexts. The discussion emphasises the need for further research to explore the influence of different cations on HS structures across a broader concentration range.