<p>Development of high-performance reverse osmosis (RO) membranes with improved permeability, selectivity, and fouling resistance is essential for sustainable water treatment. In this study, polysulfone (PSf) substrates were fabricated under different polymer concentrations, pore former contents, and phase inversion conditions to investigate their potential use as substrates for thin-film composite (TFC) membranes. The objective was to explore whether controlled phase inversion could yield membranes approaching an isoporous structure and to evaluate their influence on TFC performance. The fabricated membranes displayed a broad range of porosity (8.22–25.55%) and average pore diameter (30.1–100.6&#xa0;nm), confirming that phase inversion conditions strongly affect morphology. Although the structures did not fully meet the strict definition of isoporous membranes, tuning fabrication parameters produced more uniform and well-connected pores. Among all samples, TFC3, based on a substrate with moderate porosity (13.04%) and pore diameter (55.75&#xa0;nm), exhibited the most balanced performance, with a water flux of 29 LMH, Na₂SO₄ rejection of 97%, and excellent antifouling properties. These results demonstrate that rational substrate engineering provides an effective route to optimize TFC membrane performance for RO applications, even when complete isoporous structures are not achieved.</p>