The persistent presence of endocrine-disruptive chemicals (EDCs) in surface waters has raised serious environmental and health concerns, necessitating the development of efficient and sustainable water treatment strategies. Advanced oxidation using visible light-driven photoactive bismuth oxyiodide nanoparticles is an emerging technique for efficient water treatment. The effects of reaction parameters such as pH and temperature on the formation of semiconductor Bi _xO _yI _z nanoparticles remain underemphasized despite their critical role in tailoring size, morphology, elemental composition, specific surface area, and photocatalytic activity. Accordingly, this study aimed to develop a modified solvo-hydrothermal method to optimize the synthesis of Bi _xO _yI _z nanoparticles under varying pH and temperature conditions, and to establish correlations between their physicochemical properties – characterized by XRD, SEM, EDX, TEM, FTIR, UV-vis DRS, XPS, PL, Raman, and BET – and their photocatalytic performance. The results revealed that the sensitivity of iodine to pH and temperature significantly influenced particle growth and specific surface area, while the overall photocatalytic activity was also determined by the various phases of bismuth oxides and hydroxides formed during synthesis. It was demonstrated that the particles synthesized at pH values between 1.5 and 5.5 showed the highest photocatalytic activity due to the combined effect of larger surface area and interstitial surface defects formed due to hydroxylation. Finally, the possible configuration mechanism of the synthesized nanoparticles and the kinetics of photocatalytic degradation were discussed.