To predict the long-term behaviors of actinides in aqueous environments, complexation behaviors of actinides should be understood. Various organic ligands of chelating aromatic structure appearing in humic substances are known to form stable aqueous complexes. In this study, a benzene diol (or catechol) derivative, i.e., 4-nitrocatechol (nCA) is selected and its chemical equilibria including acid dissociation and complexation with U(VI) ion were examined using spectroscopic methods. In addition, the effect of ionic strength (Is) on those equilibria was evaluated by adjusting the level of NaClO4 in aqueous solutions. First, the experiments to determine the acid dissociation constant (pKa) of nCA were carried out in aqueous solutions with different ionic strengths from 0.01–2.0 M. The acid dissociation constants of nCA (pKa1) were measured to 6.73 ± 0.07, 6.69 ± 0.03, 6.38 ± 0.03, 6.09 ± 0.12, and 6.04 ± 0.07 at Is = 0.01, 0.1, 0.5, 1.0, and 2.0, respectively. These results were confirmed through the UV-Vis absorption spectral data analysis using the HypSpec program. As the pKa1 decreases as the ionic strength increases, except for Is = 2.0, these data were further analyzed with SIT (Specific ion Interaction Theory). Typically, as the solution becomes basic, a red shift is shown in the absorption spectrum. This effect can be understood from the intramolecular charge transfer (ICT) occurring in the deprotonated structures of nCA. At higher pH similar trends were also observed for measurement of pKa2. However, the determination of pKa2 is found not to be straightforward since a dimer formation equilibrium of nCA was observed as the ionic strength increased. This phenomenon will be discussed in detail with other supporting experimental results. Second of all, the complexation between the U(VI) and nCA in aqueous solutions was also examined. It was shown that nCA can easily form complexes with U(VI) ions at a wide range of pH via the deprotonation of their hydroxyl groups. U(VI)-nCA complexation will be further characterized by UV-Vis spectroscopy, IR and NMR by varying the solution ionic strength. The metal-ligand binding stoichiometry will be confirmed, for example, through the Job’s method. Finally, the acid dissociations constant and stability constants that were determined in this study will be used to provide species diagrams of aqueous U(VI)-nCA systems at a wide range of pH considering the effect of solution ionic strengths.