Tungsten-modified barium tantalum oxynitride is a new visible-light photocatalyst for water oxidation. In the present work, novel barium tantalum strontium tungsten oxynitride solid solutions, (BaTaO₂N)_1−x(SrWO₂N)_x, with a cubic Pmm perovskite-type structure (x = 0.01 and 0.02) have been prepared by heating oxide precursors under an ammonia flow. These (BaTaO₂N)_1−x(SrWO₂N)_x catalysts exhibited photocatalytic water oxidation activity under visible light irradiation. The crystal structure, electron-density distribution, and optical properties of (BaTaO₂N)_1−x(SrWO₂N)_x (x = 0, 0.01, and 0.02) have been studied using synchrotron X-ray powder diffraction, Rietveld analysis, the maximum-entropy method (MEM), and UV-Vis reflectance measurements. The lattice parameters of (BaTaO₂N)_1−x(SrWO₂N)_x decreased linearly with increasing SrWO₂N content x. The minimum electron density (MED) at the (Ta,W)–(O,N) bond, determined by the MEM analysis of (BaTaO₂N)_1−x(SrWO₂N)_x, increased with x, as supported by DFT-based calculations. These results indicate the formation of (BaTaO₂N)_1−x(SrWO₂N)_x solid solutions and enhanced covalent bonding due to the stronger W–N bond. The MED of the (Ta,W)–(O,N) bond was higher than that of (Ba,Sr)–(O,N), indicating that the (Ta,W)–(O,N) bond is more covalent. The presence of nitrogen in (BaTaO₂N)_1−x(SrWO₂N)_x was confirmed by the occupancy factor refined using neutron diffraction data and by the weight gain observed by thermogravimetric analysis in air. UV-Vis reflectance spectra and DFT calculations indicated that (BaTaO₂N)_1−x(SrWO₂N)_x contains W⁵⁺ cations with a [Xe] 4f¹⁴ 5d¹ electron configuration and exhibits a more n-type semiconducting character compared with BaTaO₂N, which could improve the photocatalytic water oxidation activity under visible-light irradiation.