<p>The efficiency of photoelectrochemical (PEC) water splitting is considerably controlled by the recombination of photogenerated electron/hole charge carriers at the interface. Herein, the correlation between composition and photoelectrochemical activity is studied by utilizing molybdenum-modified tungsten oxide electrodes. Molybdenum tungsten mixed oxides (Mo_<i>x</i>W_1−<i>x</i>O₃) were synthesized by <i>spray-freeze/freeze-drying</i> approach by varying <i>x</i> from 0 to 1 and studied as a photoanode. The structural changes after Mo substitution in tungsten oxide (Mo_<i>x</i>W_1−<i>x</i>O₃; 0 ≤ <i>x</i> ≤ 1.0) were observed as a function of the composition. In binary oxides, monoclinic structure (ℽ-phase) was observed until Mo substitution (<i>x</i>) reached 0.2. A coexistence of both monoclinic and orthorhombic phases was observed for <i>x</i> varying from 0.2 to 0.8. All synthesized n-semiconducting materials were photoelectrochemically active in water splitting under the acidic condition of HClO₄. The highest PEC activity was observed for the sample with low Mo content (<i>x</i> = 0.05) for which the narrowest band gap was determined. The overall activity decrease encountered for Mo-rich materials can be related to a higher tendency to photoinduced proton insertion facilitated by rhombohedral structure. The insight into the mechanism was determined by&#xa0;differential electrochemical mass spectrometry (DEMS). Oxygen (<i>m</i>/<i>z</i> 32) and hydrogen peroxide (<i>m</i>/<i>z</i> 34) were identified as main products. The material with small variation in compositions (<i>x</i> = 0.05) significantly influenced catalytic activity and selectivity, highlighting the importance of the material’s design.</p> Graphical Abstract <p></p>