<p>This work presents a comprehensive investigation of Mg_xCu_1-xFe₂O₄ (0.0 ≤ x ≤ 1.0) spinel ferrites synthesized via the sol–gel autocombustion method, with a focus on tuning their physicochemical properties for enhanced photocatalytic degradation of organic dyes. Structural analysis confirmed the formation of cubic spinels with nanometric crystallite sizes (23–28 nm) and high thermal stability. Surface area and porosity, evaluated by BET analysis, increased with Mg content, reaching 33.0 m²/g at x = 0.8, while X-ray photoelectron spectroscopy revealed systematic changes in surface redox states and oxygen species, notably the decline of Cu²⁺ and the evolution of defect-related oxygen. Optical studies showed band gap widening from 1.94 to 2.42 eV with increasing Mg substitution, reflecting changes in Cu–O and Fe–O hybridization that modulate visible-light absorption and charge carrier dynamics. Photocatalytic tests under visible light and in the presence of H₂O₂ demonstrated composition-dependent degradation efficiencies toward methylene blue (MB), rhodamine B (RhB), and malachite green (MG). Cu-rich compositions (x = 0.2–0.4) achieved the highest photocatalytic activity for MB and RhB, while Mg-rich ferrites exhibited superior performance for MG, including efficient degradation under dark conditions via a heterogeneous Fenton-like mechanism. Kinetic modeling confirmed pseudo-first-order behavior, and quantum yield analysis highlighted the importance of redox-active Cu²⁺/Fe²⁺ species and surface oxygen states in maximizing photon-to-reaction efficiency. All compositions maintained excellent structural integrity and catalytic efficiency over five reuse cycles, underscoring their stability and potential as recyclable photocatalysts for sustainable wastewater treatment.</p><p></p>