<p>This study presents a comprehensive investigation of visible-light-activated chlorine photolysis as an advanced oxidation approach for degrading the azo dye Basic Blue 41 (BB 41). The work highlights the multiple roles of inorganic and organic additives and their interactions in modulating degradation kinetics and reaction pathways. Systematic laboratory experiments have examined the influence of operational variables such as pH and light characteristics (source type, intensity, optical path), as well as components of the aqueous medium, including common inorganic anions (Cl⁻, Br⁻, HCO₃⁻, NO₃⁻, NO₃⁻, SO₄²⁻), cations (Fe²⁺, Cu²⁺, Ag⁺, Ca²⁺, Mg²⁺, Zn²⁺, Hg²⁺), and organic additives (humic substances, non-ionic surfactants Tween 20 and 80). Mixed dye systems (BB 41 + Basic Yellow 28) and realistic waters (natural mineral water, seawater, wastewater effluent) were also assessed. A pronounced light–chlorine synergy was observed, with synergy indices of 3.18 and 5.08 at pHs 7 and 9 respectively. Radical-quenching assays identified hydroxyl radicals, Reactive Chlorine Species (RCS), and ozone as the primary oxidants. Inorganic ions displayed complex, ion-specific effects, from radical scavenging (HCO₃⁻, NO₃⁻, NO₂⁻, Hg²⁺) to formation of secondary oxidant and kinetic enhancement (Cl⁻, Br⁻, Ca²⁺, Mg²⁺, Fe²⁺, Cu²⁺). Humic acid (≤ 100&#xa0;mg L⁻¹) enhanced the degradation, whereas non-ionic surfactants inhibited it. Real-matrix experiments revealed modest inhibition in mineral water, but improved kinetics in seawater and secondary effluent. Scale-relevant tests showed minor loss with extended optical paths but strong dependence on light intensity and source. Total organic carbon analysis indicated partial mineralization (~ 60% after 1&#xa0;h). These results demonstrate how synergistic interactions between inorganic and organic constituents can be harnessed alongside technical engineering parameters to optimize visible-light/chlorine Advanced Oxidation Processes AOPs for practical water treatment applications.</p>