This study investigates the selectivity of photocatalytic methane oxidation to formaldehyde (HCHO) on tungsten trioxide (WO₃), focusing on the role of different crystal facets, {001} and {110}, in determining reaction pathways. The research reveals that the {001} facet of WO₃ exhibits high selectivity for HCHO formation through an active site mechanism, where methane is directly oxidized by lattice oxygen without intermediates. In contrast, the {110} facet leads to lower HCHO selectivity due to a radical mechanism involving multiple intermediates and over-oxidation to CO₂. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), electron paramagnetic resonance (EPR), and theoretical calculations show that the competitive adsorption of methane and water, along with different activation routes on the WO₃ surface, influence the oxidation pathways. The {001} facet's lattice oxygen facilitates direct oxidation of methane to HCHO, while the {110} facet's oxygen vacancies and radical pathways lead to lower selectivity. The study also explores the effect of oxygen and water on the reaction. Increasing oxygen pressure enhances HCHO production, while higher temperatures improve the yield. The {110} facet shows higher productivity at elevated temperatures, but its lower selectivity is attributed to the radical mechanism. The {001} facet, however, maintains high selectivity due to its active site mechanism. Isotope labeling experiments confirm that the oxygen in HCHO originates from O₂, not from water. The {001} facet's lattice oxygen is crucial for HCHO formation, while the {110} facet's oxygen vacancies and radical pathways lead to lower selectivity. The study provides insights into the mechanisms of active site and radical pathways, guiding the design of efficient photocatalysts for selective methane oxidation. The findings highlight the importance of surface coordination and facet-specific properties in determining catalytic performance.This study investigates the selectivity of photocatalytic methane oxidation to formaldehyde (HCHO) on tungsten trioxide (WO₃), focusing on the role of different crystal facets, {001} and {110}, in determining reaction pathways. The research reveals that the {001} facet of WO₃ exhibits high selectivity for HCHO formation through an active site mechanism, where methane is directly oxidized by lattice oxygen without intermediates. In contrast, the {110} facet leads to lower HCHO selectivity due to a radical mechanism involving multiple intermediates and over-oxidation to CO₂. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), electron paramagnetic resonance (EPR), and theoretical calculations show that the competitive adsorption of methane and water, along with different activation routes on the WO₃ surface, influence the oxidation pathways. The {001} facet's lattice oxygen facilitates direct oxidation of methane to HCHO, while the {110} facet's oxygen vacancies and radical pathways lead to lower selectivity. The study also explores the effect of oxygen and water on the reaction. Increasing oxygen pressure enhances HCHO production, while higher temperatures improve the yield. The {110} facet shows higher productivity at elevated temperatures, but its lower selectivity is attributed to the radical mechanism. The {001} facet, however, maintains high selectivity due to its active site mechanism. Isotope labeling experiments confirm that the oxygen in HCHO originates from O₂, not from water. The {001} facet's lattice oxygen is crucial for HCHO formation, while the {110} facet's oxygen vacancies and radical pathways lead to lower selectivity. The study provides insights into the mechanisms of active site and radical pathways, guiding the design of efficient photocatalysts for selective methane oxidation. The findings highlight the importance of surface coordination and facet-specific properties in determining catalytic performance.