Polybenzimidazole (PBI) is a promising membrane material for aqueous and organic solvent nanofiltration under harsh chemical environments owing to its excellent thermal, mechanical, and chemical stability. In this study, the chemical evolution of PBI membranes exposed to concentrated aqueous inorganic acids (H₂SO₄, HCl, HBr, HI, and HNO₃) and sodium hypochlorite (NaOCl) was systematically investigated to identify simple water-based modification pathways for improving membrane stability and separation performance. Spectroscopic and morphological analyses revealed two distinct stabilization mechanisms: reversible protonation driven ionic stabilization induced by H₂SO₄ and HCl, and irreversible oxidative crosslinking induced by NaOCl. In contrast, HBr, HI, and HNO₃ promoted destabilizing halogenation or nitration related interactions. The stabilized membranes exhibited reduced molecular weight cut off values (∼870–1039 Da) together with promising Rose Bengal rejection of ∼98% in ethanol and stable solvent permeance across methanol, ethanol, and isopropanol, suggesting potential applicability in OSN separation environments. Long term DMAc exposure at 80 °C for 120 h demonstrated stable flux and rejection for the H₂SO₄ and NaOCl modified membranes. All modifications were achieved entirely in aqueous media at room temperature within 1 h, providing a simple and potentially scalable strategy for developing chemically robust PBI nanofiltration membranes.