Iron-gall inks (IGIs) are some of the most used writing materials in ancient times. Several efforts have been devoted to the identification and characterization of this type of ink in ancient manuscripts, with a particular focus on their degradation patterns. However, due to their complexity, the in-depth characterization of IGIs poses some challenges related in particular to their variability. Within this research, the main elements of variability associated with the chemical structure have been systematically investigated. The study involved the preparation of iron complexes of gallic acid, tannic acid, and polyphenols contained in oak-gall aqueous extracts, in pH-controlled regimes, and with different iron-to-ligand ratios. The so-prepared complexes have been then characterized via Raman, electron paramagnetic resonance (EPR), and infrared (IR) spectroscopies. The results obtained support the hypothesis of a strong dependence of the chemical structure on the pH condition prior to iron addition. At the same time, it has been possible to prove the efficiency of Raman spectroscopy in the in-depth characterization of historical inks and to explore the potential of IR and EPR in such studies.