In the current simulations, we assume that the charges on the polymers are fixed, independent of the pH, as if the polymers were strong polyelectrolytes. In contrast, the polymers used in our experiments, PAA and PAH, are weak polyelectrolytes, which can attain different degrees of charging, depending on pH. However, the experiments were carried out at pH ≈ 7, where the Henderson-Hasselbalch equation predicts that both ideal polycation and polyanion are fully charged, hence effectively quenched. Even though, the real system is non-ideal, both experiments on interpolyelectrolyte complexes and simulations of electrostatically cross-linked gels 111 suggest that charge regulation of interpolyelectrolyte complexes should be close to the ideal one, described by the Henderson-Hasselbalch equation. Arguably, the charge regulation can become a non-trivial actor, especially if the polyanion-polycation mixing ratio is not unity, due to the emerging Donnan potential between the phases. In principle, the G-RxMC method used here is able to quantitavely capture these affects, as repeatedly proven in simulations of gels and peptide solutions. 120-122 Herein, we focus only on the charge regulation of a small solute in the presence of interpolyelectrolyte complex. The effects of charge regulation and pH on the formation of interpolyelectrolyte complexes will be addressed in detail in our follow-up publication.