<p>Quaternary metal oxynitrides obtained by substituting oxygen anions (O^2–) in a parent oxide by nitrogen (N^3–) are encouraging candidates for photoelectrochemical (PEC) water splitting as photoanodes. A high light transmittance of photoanode thin film is promising for building bias-free tandem cells with high solar-to-hydrogen (STH) conversion efficiency. To synthetize Ta-based quaternary metal oxynitrides, metal alkoxides or metal halides with inherent moisture sensitivity and high-cost natures are most frequently used as starting sources. This work reports a general strategy to prepare semi-transparent quaternary Ta-based oxynitride thin film photoanodes based on the example of SrTaO₂N by using moisture-insensitive Ta₂O₅ as a starting material. The strategy can be adopted as a universal process for the fabrication of semi-transparent quaternary Ta-based oxynitride thin films, such as BaTaO₂N, on a large scale by using cheap chemicals.</p> Graphical Abstract <p></p>