The beneficial effect of sodium chloride on the acidic leaching of sulfide minerals is widely reported and is mostly interpreted through surface-related mechanisms. However, its influence on the oxidation-reduction potential of acidic media, a key parameter governing solution stability and oxidation thermodynamics, remains insufficiently quantified. This work quantifies the effect of NaCl concentration on ORP in NaCl–HCl and NaCl–H₂SO₄ systems. ORP was monitored together with pH, conductivity, temperature and solution composition over acidity ranges up to 2 M and salinities up to 14 wt.% NaCl. NaCl alone produced only minor changes in ORP, whereas its addition to acidic solutions increased the measured potential with a strong dependency on the acid matrix. In NaCl–H₂SO₄ systems, ORP stabilized within a moderate potential range of approximately 580–640 mV (Ag/AgCl). NaCl–HCl systems exhibited two distinct regimes: a moderate regime comparable to that observed in sulfate media and a high redox potential close to 900 mV (Ag/AgCl) under high chloride concentration and strongly acidic conditions. Comparison with 1.6 wt.% FeCl₃ (approximately 0.1 M), used as a conventional oxidant benchmark, showed comparable ORP levels, highlighting the strong influence of NaCl on the measured redox environment under specific acidic conditions. Overall, the measured ORP reflected acid-matrix-dependent mixed-potential behaviour. NaCl modified the measured ORP under acidic conditions, whether present as background salinity or as an additive. However, solution acidity or chloride concentration alone could not produce the same redox potential response. The findings provide a quantitative basis for interpreting ORP behaviour in chloride-containing acidic systems and for evaluating electrochemical conditions relevant to hydrometallurgical process development.